Method and apparatus for wire bonding

ABSTRACT

The present invention relates to a ball bonding technique using an insulated wire in assembling a semiconductor chip in which the ball is formed at the end of an insulated wire by an electrical discharge and the insulation at a portion of the wire to be bonded a predetermined distance from one end of the wire is removed by an electrical discharge.

This is a divisional application of U.S. Ser. No. 07/442,149, filed onNov. 28, 1989, now U.S. Pat. No. 5,037,023.

BACKGROUND OF THE INVENTION

The present invention relates to a technique for wire bonding and moreparticularly to a technique effective for use in a wire bonding processin assembling a semiconductor integrated circuit device using aninsulated wire.

In the fabrication of a semiconductor integrated circuit devices, theprocess of wire bonding is known which wires conductive metal wires forconnecting a plurality of electrodes for external connection disposed ona semiconductor chip on which predetermined integrated circuits areformed to a plurality of leads functioning as terminals of externalconnection when the semiconductor chip is mounted.

To meet the increasing demands of late for higher packaging density andsmaller size of semiconductor integrated circuit devices, the density ofthe externally connected electrodes is increasing greatly, and hence,the spacing between the bonding wires and diameter of the bonding wiresis decreasing. As a result, problems arise such as the occurrence ofshort circuiting between bonding wires and abnormalities due todeterioration in rigidity of the bonding wire.

To cope with such problems, it is known in the art of wire bonding touse an insulated wire formed of a metallic core wire covered with aninsulating film.

In the well known ball bonding technique, the end of a wire passedthrough a bonding tool such as a capillary is melted and formed into aball and then bonded to a contact area. But in the bonding of the wireto the side of a lead, an area of the peripheral surface of the wirecovered with an insulating film is compression-bonded to the surface ofthe lead. Therefore, it is expected that the bonding strength islowered, the electric resistance is increased, and reliability of thebonding is lowered.

In view of these problems, there have been disclosed improved methodsfor wire bonding using an insulated wire, particularly for the bondingto the side of the lead, in Japanese Laid-open Patent Publication Nos.62-140428 and 62-104127.

The former improved method is such that the pressure provided throughthe bonding tool, when an insulated wire is bonded to the lead side, isprogressively increased in multiple steps so that the nonconductiveinsulating film existing between the core wire of the insulated wire andthe lead is removed and thereby a reliable bond is obtained.

The latter improved method is such that when a capillary with theinsulated wire is passed therethrough and an insulated wire is bonded tothe lead side, the wire is heated and also given vibration so that theinsulating film existing between the core wire of the insulated wire andthe lead is removed and a reliable bond is obtained.

SUMMARY OF THE INVENTION

In accordance with the invention, when an insulated wire is bonded tothe lead, the bonding is started as with the above described prior withthe insulating film existing between the materials to be bondedtogether. According to studies made by the present inventor, there is atheory that foreign materials are produced from the insulating filmbeing thermally changed in quality which remain between the core wireand the lead which results in deterioration in the bonding strength or aincrease in electric resistance that prevents a reliable bond from beingobtained.

Since in both of the above described methods, bonding is performed withthe insulating film existing between the core wire of the insulated wireand the bonding tool, pieces of the insulating film peeled off the wireor other foreign materials produced by the applied bonding heat willcoat the wire insertion portion of the bonding tool, causingcontamination which prevents the insulated wire from being smoothlydelivered and pulled back therethrough. Thus, achievement of stabilizedbonding becomes difficult.

The present invention was made in view of the above mentioned problemsand has the following objects.

A first object is to provide a wire bonding process whereby a reliablebond can be produced between an insulated wire and a lead without havingan apparatus of complex structure.

A second object is to provide a wire bonding process in whichcontamination of the bonding tool resulting from the insulating film ofthe insulated wire is prevented so that stabilized wire bondingoperation can be performed.

A third object is to provide a wire bonding process with stabilizedoperation and pull back the wire.

A fourth object of the present invention is to provide a wire bondingwith a bonding ball in which formed stabilized ball formation isachieved at all times.

A further object of the present invention is to provide during a wirebonding process a reliable bond between an area of the peripheralsurface of an insulated wire and a lead.

The above and other objects and novel features of the present inventionwill be made more apparent from the description given hereinbelow andthe accompanying drawings.

Representative aspects of the present invention disclosed herein aregenerally summarized as follows.

First, an insulated wire of a length determined to be required by acalculation based on position information is drawn out of the front endof the bonding tool, a discharge is produced between the core wire ofthe insulated wire at a second portion to be bonded and dischargingelectrode through the insulating film so that an area of the insulatingfilm is removed by energy of the discharge to expose a core of the wire.Thereafter a discharge is produced between the core wire at a front endof the insulated wire at a first portion to be bonded and the aforesaiddischarging electrode so that insulation at the front end is removed andthe core is melted into a ball. Thereafter, the ball is bonded to afirst position and subsequently, the exposed portion of the insulatedwire drawn out of the bonding tool is bonded to the second position.

To attain the above feature, the apparatus is provided with adischarging electrode movable between a position immediately below anend of the insulated wire at a first portion of the wire to be bonded ofthe insulated wire passed through the bonding tool and a position closeto a peripheral surface of the insulated wire at a second portion of thewire to be bonded. Further, at the time of the discharging, a gas isblown to the surface of the discharging electrode.

An urging force is imposed on the wire at its portion between a spooland the bonding tool in a direction substantially orthogonal to a radialdirection of the wire so that tension is applied to the wire in adirection opposite to a direction of movement toward a bonding position.Simultaneously, the condition of the wire at the position where theurging force is imposed is observed by a detection to detect aslackening of the wire, and rotation of the spool is controlledaccording to the result of the detection so that the slackening of thewire may be kept at a constant state at all times.

During the course of discharging for formation of the ball or removal ofthe insulating film, the voltage drop, which varies with decrease in thelength of the insulated wire wound on a spool of wire as the bondingprocess of a spool of wire proceeds, is detected and a detected voltagedrop is added to a voltage drop across a discharge gap between thedischarging electrode and a front end or the second portion of the wireto be bonded of the insulated wire to calculate the voltage to beapplied in the time of the next discharge to produce a ball for asubsequent ball bond. The values of voltage drop across the dischargegap are obtained as a function of discharge gap and discharge current.

The bonding tool is horizontally moved and lowered to the secondposition after the first portion to be bonded of the wire fed from thewire spool and passed through the bonding tool has been bonded to thefirst position. The wire is then set free from clamping by the bondingtool, the bonding tool is elevated to a predetermined height and tensionis applied to the wire in the direction opposite to the second position.

The apparatus for wire bonding is provided with a clamper disposed inthe path of the wire between a wire spool and the bonding tool forclamping the insulated wire at its periphery. The clamper iscontrollable so that it provides at least two steps of clamping force toproduce fixed clamping and frictional clamping.

After a bonding process of a last wire for a preceding semiconductorchip has been finished, the insulation of the following insulated wireis removed at a second portion of the wire to be bonded at a fixeddistance from an end thereof and a dummy bonding is performed to afollowing semiconductor chip or mounting member and subsequently abonding process for bonding the first wire to a following semiconductorchip is performed.

According to the above described first aspect of the present invention,since the same discharging electrode is used both for forming the balland for removing the insulation from the wire, the ball formation andthe insulation removal at the first portion to be bonded and the secondportion to be bonded are made possible without having an apparatus ofcomplex structure. Therefore, wire bonding providing highly reliablebonds with the use of an insulated wire can be achieved. An arrangementto achieve this objective can be easily embodied by constructing thedischarging electrode to be movable between the position immediatelybelow the insulated wire at the first portion to be bonded of theinsulated wire and a position close to the periphery of the insulatedwire at the second portion to be bonded of the insulated wire. Further,by blowing a gas to the surface of the discharging electrode at the timeof discharging, the discharging electrode is prevented from beingcontaminated to provide a stabilized discharge over a long period oftime.

According to the second aspect of the invention, since, the wiretensioning and wire detector means are operative at the same positionand at the same time, application of tension to the wire and detectionof the wire can be performed at the same position and at the same time.Hence, appropriate control of the rotation of the wire spool can beachieved and the wire can be maintained at a state of constantslackening at all times. Thus, no fluctuation is produced in the tensionapplied to the wire above the bonding tool and a stabilized bondingoperation can be performed.

According to the third aspect of the invention, since an optimum valueof applied voltage adjusted with decreasing length of the insulated wirecan be supplied at all times, it is possible to form balls of astabilized size and to remove the insulating film in a stabilized lengthalong the insulated wire. Therefore, highly reliable and stabilized wirebonding can be achieved regardless of the length of the insulated wirewound around the wire spool.

According to the fourth aspect of the invention, since the height of aformed wire loop between the semiconductor chip and a lead frame is madecontrollable, the length of the wire for forming the wire loop can besteadily controlled. Therefore, the second portion to be bonded (theportion where the insulating film is to be removed) can be set with highprecision. Further, since the apparatus is provided with a clampercontrollable having at least two steps of clamping force providing fixedclamping and frictional clamping, elevation of the wire with the clamperand the height of the wire loop are made controllable. Thus, thestretched condition of the wire is made controllable without increasingthe number of the clampers.

According to the fifth aspect of the invention, since the insulation isremoved at the second portion to be bonded a fixed wiring distance fromthe first portion when the wire is to be bonded to a semiconductor chip,a dummy bonding can be performed using the wire. Therefore, a stabilizedand appropriate wiring distance is assured for the subsequent bondingusing the first wire and the following wires.

Other representative aspects of the invention disclosed herein aresummarized as follows.

A method for wire bonding according to the present invention comprisespassing an insulated wire passed through a bonding tool, bonding a frontend of the insulated wire to a first position, bonding an area of aperipheral surface of the insulated wire drawn out of the bonding toolto a second position to achieve an electric connection between the firstposition and the second position. The insulated wire is drawn out of afront end of the bonding tool a required length determined by acalculation based on the first and second positions, a discharge isproduced between a core of the wire at a portion to be bonded to thesecond position through the insulation to a discharging electrode sothat the insulation is removed by energy of the discharge and an exposedportion of the core is formed and the exposed portion is bonded to thesecond position.

The apparatus for wire bonding according to the present inventioncomprises a bonding tool with an insulated wire passed therethrough andmovable in three-dimensions relative to an object and a dischargingelectrode for producing discharges between an insulated wire and theelectrode for bonding a front end of the wire where a ball is formed bya discharge to a first position and bonding an area of a peripheralsurface of the insulated wire drawn out of the bonding tool to a secondposition to electrically connect the first position and the secondposition. The required length of the insulated wire to be wired betweenthe first and second positions is calculated based on information onindividual first and second positions, the insulated wire is drawn outof the front end of the bonding tool the required length based on theresultant calculation, a discharge is produced between the core of theinsulated wire at the portion to be bonded to the second positionthrough the insulation to the discharging electrode so that an exposedportion of the core is formed by removal of the insulation by energy ofthe discharge and the exposed portion is bonded to the second position.

According to the method for wire bonding as described above, since theinsulation of the wire at a portion to be bonded is removed and theexposed portion of the core wire is formed before an area of theperipheral surface of the insulated wire is bonded to the secondposition, it is possible to perform the bonding operation with the coreexposed at the exposed portion and the second position by directcontact. Therefore, lowering of the reliability of the bonded portiondue to the insulating film coming between the core and the secondposition is eliminated. Therefore, the reliability on the bond betweenthe periphery of the insulated wire and the second position can besecured.

Further, since the insulation does not come between the core and thesecond position, foreign materials resulting from peeling off orthermally produced change in the quality of the insulation is greatlyreduced. Since the bonding tool presses the core of the insulated wireexposed at the exposed portion directly to the second position, theforeign materials do not contact the bonding tool, providing smoothpassage of the insulated wire through the bonding tool and stabilizedcontinuation of the bonding operations is made possible.

Further, according to the apparatus for wire bonding according to thepresent invention, by suitably independently controlling first andsecond clampers capable of moving independently of each other deliveryof the insulated wire from the bonding tool can be controlled. Theexposed portion of the core can be formed by removing the insulation atthe portion to be bonded to the second position at a desired distancefrom the end of the insulated wire prior to the bonding of theperipheral portion of the insulated wire to the second position. Thus,the bonding operation can be performed with the core exposed at theexposed portion and the second position in direct contact. Accordingly,the lowering of the reliability on the bonded portion due to theinsulating film coming between the core wire and the second position canbe eliminated and the reliability on the bond between the periphery ofthe insulated wire and the second position can be achieved.

Further, since no insulation is disposed between the core wire and thesecond position, production of foreign materials resulting from peelingoff or thermally caused change in quality of the insulation can begreatly reduced. In addition, the bonding tool can press the core of theinsulated wire exposed at the exposed portion formed in advance directlyto the second position, without foreign material contacting the bondingtool providing smooth passage of the insulated wire through the bondingtool and stabilized continuation of the bonding operations is madepossible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing an apparatus for wire bondingas an embodiment of the present invention;

FIGS. 2a-2j are an explanatory diagram showing positional relationshipsamong the bonding tool and other elements according to steps in the wirebonding process;

FIG. 3 is an explanatory diagram showing timing of operation in eachmechanism corresponding to the above bonding process;

FIG. 4 is an explanatory diagram showing a semiconductor chip and itsperiphery finished with the bonding according to the present embodiment;

FIG. 5 is an explanatory diagram showing a relationship between a heightof a wire loop and the wiring distance in the first embodiment;

FIG. 6 is an explanatory diagram showing a relationship between a lengthrequired for ball formation and ball diameter in the first embodiment;

FIG. 7 is a flow chart showing a bonding process according to the firstembodiment;

FIG. 8 is a perspective view showing relative position between an airblow nozzle and a discharging electrode;

FIG. 9 is a perspective view showing a driving mechanism of adischarging electrode;

FIG. 10 is a sectional view showing relative position between adischarging electrode and an insulated wire;

FIG. 11 is a plan view showing a clamping mechanism for a secondclamper;

FIG. 12 is a perspective view showing a wire tensioning portion;

FIG. 13 is a sectional view showing a wire detection mechanism;

FIG. 14 is a perspective view showing a wire spool;

FIG. 15 is a partially sectional view of a mounting mechanism of theabove wire spool;

FIG. 16 is a block diagram showing a circuit configuration of adischarge power circuit;

FIG. 17 and FIG. 18 are explanatory diagrams showing relationshipbetween voltage drops across a coil portion and a discharge gap and theapplied voltage;

FIG. 19 is a schematic diagram for explaining discharge conditions forremoving an insulating film;

FIG. 20 is an explanatory diagram showing an example of gap voltagedrops obtained from results of experiments;

FIG. 21 is an explanatory diagram showing relationship between variationin gap voltage drop and discharge current;

FIG. 22 is an explanatory diagram showing a state where a voltage fordielectric breakdown is applied before a discharge is produced forremoving insulating film;

FIG. 23(a) to FIG. 23(j) are process diagrams showing an example ofoperations in another embodiment of the invention;

FIG. 24 is a side view schematically showing an arrangement of anotherembodiment of an apparatus for wire bonding of the present invention;

FIG. 25 is a perspective view of the major portion taken out of anapparatus for wire bonding as another embodiment of the presentinvention;

FIG. 26 is an enlarged sectional view showing a portion of the above;and

FIG. 27(a) to FIG. 27(h) are explanatory drawings showing an example ofa sequential manufacturing method of wire bonding as an additionalembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1

An apparatus for wire bonding of the present embodiment will bedescribed with reference to FIG. 1. On bed plate 1, a bonding stage 2 ispositioned with its longitudinal direction into the drawing. On thebonding stage 2, lead frame 4 is a mounting member. In the center of thelead

frame 4, a tab 4a is formed and on the tab 4a there is fixed, with aconducting adhesive such as resin paste, (not illustrated), asemiconductor chip 3, which is to be heated to a predeterminedtemperature by means of a heater 2a provided within the bonding stage 2.Above the bed plate 1 and at a side of the bonding stage 2, an X-Y table5 is movable on a horizontal plane. On the X-Y table 5, there issupported a bonding head 6 through pivot 7 for rotating in a verticalplane with one end positioned over the bonding stage 2. The other end ofthe bonding head 6 is controlled for vertical movement by means of alinear motor 8 fixed to the X-Y table 5. At the end of the bonding head6 on the side toward the bonding stage 2, a bonding arm 9 ishorizontally supported with a end located right above the bonding stage.At the capillary 10 is positioned as a bonding tool made of ruby,ceramic, or the like. The capillary 10 is fixed in place, with a wireinsertion hole, (not illustrated), axially passing through the capillaryin the vertical direction. Into the wire insertion hole of the capillary10 is inserted an insulated wire 13 fed from a wire spool 12 through awire tensioning device 22, a wire guide 21, a second clamper 15, and afirst clamper 14. At the side of the base of the bonding arm 9, anultrasonic vibrator 11 is positioned comprising a piezoelectric elementor the like. The vibrator 11 when necessary is capable of applyingultrasonic vibration, for example, at 60 kHz and with amplitude of 0.5μm-2.0 μm to the capillary 10 fixed to the front end of the bonding arm9.

The above described bonding head 6 is arranged to be controlled by acontrol unit 20 including a CPU and a memory unit, (not illustrated).The control is performed, for example, in such a way that the drivingvoltage of the linear motor 8 is servo-controlled according to outputsignals of a speed detecting means and a position detecting means (notillustrated) for detecting the movement of the bonding head 6. Further,the bonding load at the time of bonding on the semiconductor chip 3 andthe lead frame 4 has the driving current of the linear motor 8controlled. Above the bonding head 6, a recognition device 19 isdisposed fixed to the X-Y table 5. The recognition device 19 includes,for example, a TV camera and performs a function of detecting thebonding position on the semiconductor chip 3 and the lead frame 4. Thecontrol unit 20, in response to image information from the recognitiondevice 19, gives instructions to the bonding head 6 to bond insulatedwires 13 to detected points on the semiconductor chip 3 and detectedpoints on the lead frame 4 thereby connecting these points with eachother in succession.

The insulated wire 13 with reference for example to FIGS. 2(a)-2(g) hasa core wire 13a as a conductor and an insulating film 13b made of highpolymer resin having an electric insulating property applied to theperiphery of the core wire. As the core wire 13a, a gold (Au) wire of adiameter, for example, of 20-50 μm or, preferably, 25-32 μm is used. Theinsulating film 13b is made, for example, of polyurethane, polyester,polyamideimide, polyesterimide, or nylon. It should preferably be madeof the above mentioned polyurethane or heat resisting polyurethane,i.e., polyurethane treated so as to exhibit heat resistance. Thethickness of the insulating film may range from 0.2 μm-5.0 μm with athickness between 0.5 μm-2.0 μm being preferred. The method for coatingthe wire with such an insulating film 13b is by dipping the core wire13a in a solution of the resin material as described above diluted witha solvent to 5- 20% of concentration and then drying the wire by heat.To suppress production of pin holes in the insulation by this process,it is preferred to repeat the coating and drying several times. To bemore specific, the production of pin holes in the insulation wassubstantially reduced by repeating the coating and drying 5-15 times.

With reference to FIG. 14, an insulated wire 13 of a length, for exampleof 100-1000 m, is wound around the wire spool 12 and end 13h isconnected with a conducting portion of the wire spool 12. The wire spool12 is electrically connected with a spool holder 25 (FIG. 1) and thespool holder 25, in turn, is connected with a discharge power circuit18. The wire spool 12 is made of a conducting metal such as aluminum(Al) and the base end 13h of the insulated wire 13 is stripped of itsinsulating film. A gas burner (not illustrated) may be used to heat theinsulating film 13b to thereby thermally decompose and remove it tostrip the wire 13 to explore the core 13g. At this time, the base end ofthe core wire 13a itself may be heated to be formed into a ball.Otherwise, to improve reliability of the electrical connection aplurality of such balls may be formed along the core wire 13a. The baseend where the core wire 13a has been exposed as described above is fixedto the end portion of the wire spool 12 with an adhesive tape or thelike. Thus, the potential of the base end of the core wire 13a of theinsulated wire 13 is kept at the same potential as that of the wirespool 12.

FIG. 15 illustrates a mounting mechanism of the wire spool 12. The wirespool 12 is put in a spool holder 25 and is further fixed to the spoolholder 24 by means of a spool fixing member 252 so as to be secured inplace. The spool holder 25 is coupled with a cylinder 26a extended frommotor 26 retained by an L-shaped retaining member 254 fixed to the bedplate 1, so that the wire spool 12 together with the spool holder 25 maybe controllably rotated. An L-shaped plate spring 253 has a back endfixed to an electrode terminal 255 disposed on the retaining member 254.The front end of the plate spring 253 is arranged to urge the spoolholder 25 in an axial direction opposite to the cylinder 26a. Theelectrode terminal 255 is connected to the ground (GND) side of thedischarge power circuit 18. By virtue of the arrangement shown in FIG.15, i.e., the wire spool 12, spool holder 25, plate spring 253, and theelectrode terminal 255, the core wire 13a of the insulated wire 13 iskept at the same potential as the potential GND of the discharge powersupply circuit 18. With reference to FIG. 1 the insulated wire 13 fedfrom the wire spool 12 is given a predetermined tension and the tensionis detected by a wire tensioning device 22.

The structure of the wire tensioning device 22 is described below withreference to FIG. 12. The wire tensioning device 22 has a pair of airblow plates 22a, supported by a support portion 22d at a predeterminedspacing. A gas is supplied from an air supplying inlet 23 which passesthrough the space between the confronting air blow plates at apredetermined fluid pressure. The insulated wire 13 is passed throughthe space between plate 22a in a direction perpendicular to thelongitudinal direction of the air blow plates 22a, to cause theinsulated wire 13 to be urged by the fluid pressure of the supplied gasin a direction opposite to the air supplying inlet 23 causing theinsulated wire 13 to be subjected to a predetermined value of tension.

In the principal plane of the above described air blow plates 22a thereis a circular detection hole 22b. In the detection hole 22b, there isinserted the front end of a reflection type optical fiber sensor 24functioning as a light detection means.

FIG. 13 is a sectional view illustrating the wire detection mechanism inthe wire tensioning device 22 in more detail. The optical fiber sensor24 comprises a light emitting fiber 241a and a light receiving fiber241b. The fibers 241a and 241b are each formed in an optical fiber cable242 of the same structure. The light emitting fiber 241a is connectedwith a light source such as an LED (not illustrated). The lightreceiving fiber 141b is connected with a photodetector device such as aphototransistor. Thus, a detecting light beam emitted from the end ofthe light emitting fiber 241a is reflected by the peripheral surface ofthe insulated wire 13 and the reflected light beam is detected by thelight receiving fiber 241b.

Representing the distance from the end of the optical fiber sensor 24 tothe inner face of the air blow plate 22a opposite thereto by δ1, thedistance from the end of the optical fiber sensor 24 to the inner faceof the air blow plate 22a closer thereto by δ2, the distance between theopposing faces of the air blow plates 22a, by δ3, and the distancebetween the end of the optical fiber sensor 24 to the outer face of theair blow plate 22a opposite thereto by δ4, with δ1=0.4 mm, δ2=0.1 mm,and δ3=0.3 mm, and further, with δ5>2 δ4 tan 30°, it is possible todetect insulated wires 13 having a diameter as small as 15 δm. The abovementioned values are just for exemplification. These values can besuitably changed according to the light propagating characteristic ofthe optical fiber cable 24a, the diameter of the fiber, and the like.That is, they can be changed at will within tile limits enabling thedetection of the insulated wire 13.

The hole 22c of the diameter δ5 provided on the opposite side of theoptical fiber sensor 24 is that provided for preventing the emittedlight beam by the light emitting fiber 141a from being reflected by theinner face of the air blow plate 22a on the opposite side, therebycausing the optical fiber sensor 24 to malfunction. Instead of providingthe hole 22c, the inner face of the air blow plate 22a may be coloredblack so that the detecting light beam is absorbed thereby and no lightbeam is reflected therefrom. The light emitting fiber 141a and the lightdifferent directions.

In the wire tensioning device 22 of the described arrangement of FIG.13, the insulated wire 13 is constantly provided with a predeterminedvalue of tension by means of fluid pressure of the supplied gas suppliedbetween the air blow plates 22a. As the supply gas, clean air, passedthrough a filter or the like can be used. Preferable a flow rate is usedof 5-20 liters per minute. If the flow rate is smaller than the aboveflow, the insulated wire 13 slackens between the first clamper 14 andthe second clamper 15 of FIG. 1. As a result of slackening it becomesdifficult to properly control the insulated wire 13. On the other hand,if the flow rate is higher than the above flow, the tension pulling thewire upward becomes too great, whereby not securing a suitable wireloop, making an accurate tail cut at the time of second bondingdifficult, which can cause the insulated wire 13 to be broken.

In the wire tensioning device 22, the optical fiber sensor 24 alwaysmonitor the condition of the insulated wire 13 for slackening. Moreparticularly, if the reflected beam of light from the insulated wire 13is detected by the optical fiber sensor 24, then it is determined thatit has less than a predetermined value of slackening, or that it has ina tensed condition. When the tensed condition is detected, the shaft ofmotor 26 coupled with the spool holder 25 is rotated a predeterminedamount so that the insulated wire 13 is fed from the wire spool 12 apredetermined length. Thus, the insulated wire 13 above the wire guide21 can be maintained in a state having a predetermined degree ofslackening.

Since the tensioning of the insulated wire 13 and detection of theinsulated wire 13 are performed at the same position and time by thewire tensioning device 22 integral with the air supply, the rotation ofthe wire spool 12 can be suitably controlled and the slackening of theinsulated wire 13 can always be maintained in a predetermined condition.Thus, the tension above the capillary 10 is kept free from fluctuation,and thereby, stabilized bonding operation is assured at all times.Further, since the detection of the insulated wire 13 can be conductedin the manner not contacting the insulated wire 13 by the use of theoptical fiber sensor 24, the insulated wire 13 is not damaged, andinsulation and strength of the insulated wire 13 in the path of supplyto the bonding stage can be prevented from being lowered. Further, byvirtue of the structure of the wire tensioning device 22 being integralwith the air supply as described above, providing a separate detectionmechanism of the insulated wire 13 can be eliminated and therebysimplifying of the structure of the device.

With reference to FIG. 1, the insulated wire 13 which passes through thewire guide 21 and which is aligned thereby is lowered through the secondclamper 15, the first clamper 14, and passes through the capillary 10.The first clamper 14 is arranged to be fixed to the bonding head 6 andmovable up and down in synchronism with the bonding arm 9. Althoughdetails of tho clamping portion of the first clamper 14 are not shown,it is located right above the capillary 10 and its clamping load iscontrolled to be 50-150 g. The second clamper 15 is fixed to the X-Ytable 5 and located right above the first clamper 14 at a height thatdoes not interfere with the vertical movement of the first clamper 14.The second clamper 15 has a mechanism enabling it to be opened andclosed independently of the first clamper 14.

The clamping mechanism of tile second clamper 15 which is one of thefeatures of the first embodiment is described with reference to FIG. 11.The second clamper 15 includes clamper chips 151a and 151b havingconfronting faces made of ruby or the like. The insulated wire 13 isclamped and unclamped by opening and closing operations of the clamperchips 151a and 151b. The clamper chip 151a on one side is fixed to aswing arm 156. The swing arm 156 is arranged to be rotatable around asupporting shaft 157 having rear end urged in the opening direction bymeans of a compression coil spring 155 attached to a supporting member158. Between the rear end of the swing arm 156 and the supporting shaft157, a solenoid 153a, in the off state causes the rear end of the swingarm 156 to be opened state by the expanding force of the compressioncoil spring 155 and the front end of the clamper chip 151a is in aclosed state which clamps the insulated wire 13. Conversely, when thesolenoid 153a is in the on state, a rod 154a of the solenoid 153a ismoved to the left whereby the insulated wire 13 is released from theclamped state. The clamper chip 151b is attached to a plate spring 152aprojecting from the supporting member 158. The clamper chip 151b isurged from its rear side to its chip face by the front end of a rod 154bof a solenoid 153b. The solenoid 153b is illustrated being in the onstate, when the plate spring 152a is restricted by the rod 154b so asnot to function as a plate spring. The rod 154b is provided with anL-shaped plate spring 152b having one end fixed to the supporting member158. The rod 154b, when the solenoid 153b is in the off state, is urgedto the right. Therefore, when the solenoid 153b is turned off, the platespring 152a for retaining the clamper chip 151b is restored to itsoriginal function as a plate spring.

In this way, the clamper chip 151b can exhibit two steps of clampingaction. The first being an urging force of the plate spring 152a and thesecond being by the rod 154b of the solenoid 153b. Thus, the secondclamper 15 is enabled to exercise two functions, one as a fixed clamperfor fixedly clamping the insulated wire 13 and the other as a frictionalclamper for clamping the insulated wire 13 in a predetermined frictionalstate.

The control of the clamping force of the second clamper 15 is describedas follows.

1. At the time when unclamped

At this time, one solenoid 153a is put into the on state, the rod 154ais moved to the right against the compression coil spring 155, the endof the swing arm 156 is opened, and the clamper chip 151a is moved awayfrom the insulated wire 13. Meanwhile, the solenoid 153b is in the offstate and the rod 154b is moved to the right by the L-shaped platespring 152b. Therefore, the insulated wire 13 is held free between theclamper chips 151a and 151b.

2. At the time when the first clamping load is imposed

This is a state of so-called "frictional clamping". At this time, opensolenoid 153a is put into the off state so that the compression coilspring 155 urges the rear end of the swing arm 156 is open. The clamperchip 151a at the front end of the swing arm 156 moves toward theinsulated wire 13. The distance of the movement at this time isrestricted by a stopper, (not illustrated). The other solenoid 153b isin the off state and the rod 154b is moved to the right by the L-shapedplate spring 152b. The clamper chip 151a is under the urging force ofthe compression coil spring 155, while the other clamper chip 151b underthe urging force of the plate spring 152a. At this time, by suitablyadjusting the resilient force and displacement of the plate spring 152a,the clamping load on the insulated wire 13 can be set to a light load.In this case, the insulated wire 13 is not fixedly held by the secondclamper 15, and is fictionally fed between the clamper chips 151a and151b when the insulated wire 13 is given a force pulling it out of thecapillary 10. Since, the tension breaking of the insulated wire 13, witha diameter is 30 μm is around 12-16 gf, the frictional force ispreferably held to be lower than that, for example, around 1-4 gf. Whenthe coefficient of friction is assumed to be approximately 0.2, thefrictional force around 1-4 gf as described above corresponds to theclamping force around 5-20 gf.

By producing such a "frictional clamping" state in the later describedwire bonding (refer to the description of FIG. 2(f)), the second clamper15 can be used as a loop controlling clamper for controlling the heightof a wire loop. Therefore, there is no need for providing a loopcontrolling clamper separately in the present apparatus. The secondclamper 15 serves dual functions of pulling up the wire (at fixedclamping; refer to FIG. 2(j)) and controlling the height of a wire loop(at frictional clamping; refer to the description of FIG. 2(f)).

3. At the time when the second clamping load is imposed

This is a state of so-called "fixed clamping". When the solenoid 153b isfirst put into the on state, the rod 154b is moved to the left againstthe urging force of the L-shaped plate spring 152b. Thereby, the platespring 152a is restricted in its elastic deformation. Then, the solenoid153a is put into the off state and the compression coil spring 155 urgesthe rear end of the swing arm 156 to open. The clamper chip 151a at thefront end of the swing arm 156 is moved toward the insulated wire 13.Since the plate spring 152a for supporting the other clamper chip 151bis restricted in its elastic deformation by the rod 154b, the clampingload at this time is determined according to the urging force of thecompression coil spring 155. If the clamping load resulting from thecompression coil spring 125 is set to 50-150 gf and the urging load ofthe rod 154b by electromagnetic force of the solenoid 153b is set to 300gf, the urging force of the compression coil spring 155 can beeffectively transmitted to the insulated wire 13.

Although solenoids 153a and 153b and plate springs 152a and 152b wereused as the driving mechanism of the second clamper 15 in the abovedescription, actuators such as rotating motors and linear motors may beused instead of the solenoids 153a and 153b, and tension coil springs orthe like may be used instead of the compression coil spring 155 andplate springs 152a and 152b. The clamping load by the clamper is madechangeable according to the purpose and use. The insulated wire 13passed through the first clamper and second clamper 15 is further passedthrough the capillary 10 such that the front end 13e (FIG. 2(a) of thewire sticks out of the front end of the capillary 10.

Referring to FIG. 1, an air blow nozzle 16 and a discharging electrode17 is positioned under the capillary 10. The air blow nozzle 16 is forblowing a gas to the electrode surface of the discharging electrode 17as illustrated in FIG. 1 at the time of discharging to thereby preventthe electrode surface from being contaminated by the gas produced bythermal decomposition of the insulating film 13b. The air blow nozzle 16is fixed to the X-Y table 5 an(i structured so as to be able to performair blowing to the position set at a height (L₀ or L₃) right under thecapillary 10. More particularly with reference to FIG. 8, the air blownozzle 16 includes a nozzle tube 16a for conducting gas (air) suppliedfrom a gas supply inlet 16b therethrough and the nozzle tube 16a has atits front end a gas blow outlet 16c having an opening with a reducedsectional area for raising the blowing pressure. The blowing height L₁₇of the air blow nozzle 16 referenced from the lead frame 4 is desired tobe in the middle of the heights L₀ and L₃ of the electrode surfaces ofthe discharging electrode 17. Therefore, such 8 height L₁₇ can becalculated from

    L.sub.17 =(L.sub.0 +L.sub.3)/2

As an example, a good result was obtained by arranging the sectionalarea of the gas blow outlet 16c to be 0.2-1.0 mm², blowout flow to be0.1-0.5 l/min, and the distance between the gas blow outlet 16c and theelectrode surface to be 0.5-2.0 mm.

When the air flow is much higher than the above value, the dischargespark becomes unstable, and sometimes formation of the ball 13c becomesdifficult or satisfactory removal of the insulating film 13b becomesunachievable. Conversely, when the blowout rate is too small, itsometimes becomes difficult to effectively prevent the electrode surfacefrom being contaminated.

Although air was used in the above as the gas to be blown out, inert gassuch as argon (Ar) and nitrogen (N2) or other gases may be used.

With reference to FIG. 9, structure of the discharging electrode 170disposed opposite to the air blow nozzle 16 is described. Thedischarging electrode 170 has as its discharging terminals an electrodepiece 170a and an electrode piece 170b. While the former electrode piece170a is an electrode for an exclusive use to remove the insulating filmthe latter electrode piece 170b functions as is an electrode both forremoving the insulating film 13b and for forming the ball. The electrodepiece 170a is structured such that it is sandwiched at its top andbottom faces in between upper and lower insulator pieces 170c made of anelectric insulating material with these elements being supported by anelectrode arm 174a. The electrode 170a and 170b have cross-sections suchthat angles facing each other are acute angle, that a concentrateddischarge spark may be readily produced between each electrode piece andthe core wire 13a at the time or removal of the insulating film 13b. Theelectrode piece 170b, the same as the electrode piece 170a, is arrangedsuch that it is sandwiched at its top and bottom faces in betweeninsulator pieces 170d. The upper side is constructed such that thedischarging surface is exposed to the outside. The exposed portionfunctions as the electrode surface for ball formation. The electrodepieces 170a and 170b can be made of heat resisting electric conductingmaterial such as tungsten (W) and the insulator pieces 170c and 170d canbe formed of an insulating material such as ceramics. For fixing theelectrode piece 170a, 170b with the insulator pieces 170c, 170d, a heatresisting adhesive such as ceramic bond can be used. The electrodepieces 170a and 170b and insulator pieces 170c and 170d, respectively,are connected with swing arms 173a and 173b through the electrode arms174a and 174b for rotation around an axis of rotation 171.

The swing arm 173a is connected at its end opposite to the end where theelectrode arm 174a is formed with a first solenoid 172a for dischargingan electrode fixed to a supporting portion 175, and the swing arm 173bis connected with a second solenoid 172b for discharging the electrode.Both the swing arms 173a and 173b are urged upward and to the right bytension coil springs 176a and 176b. Operating mechanisms in thedischarging electrode 17 will be described below.

1. When discharging operation is not performed

At this time, the first solenoid 172a for discharging the electrode isin the off state. The electrode piece 170a is pulled under the urgingforce of the tension coil spring 176a locked to the swing arm 173a inthe direction away from the insulated wire 13 and stopped at apredetermined position by means of a stopper (not illustrated).

The second solenoid 172b for discharging the electrode is in the onstate. The electrode piece 170b is spaced away from the insulated wire13 by the electromagnetic force of the . . . second solenoid 172b fordischarging electrode imposed the swing arm 173b.

2. When a ball is formed

At this time, the second solenoid 172b for discharging the electrode isfirst brought into the off state. The tension of the tension coil spring176b is imposed oil the swing arm 173b and moves it toward the insulatedwire 13. At this time, the electrode piece 170b stops at the positionright under the front end (lower end) of the insulated wire 13 by meansof a stopper (not illustrated). For simplicity, it is shown in FIG. 9 asif the discharging electrode 170 is moved up and down with respect tothe insulated wire 13, but in reality, the height of the dischargingelectrode is fixed and the insulated wire 13 is vertically moved bymeans of the capillary 10 and the first clamper 14. It is desired, atthis time, that the position of the stopper is adjusted so that the ballforming electrode surface (exposed surface) of the electrode piece 170bcomes to the position best suited for performing the dischargingfunction.

3. When insulating film is removed

At this time, the first solenoid 172a for discharging electrode is firstbrought into the on state. The swing arm 173a is pulled in byelectromagnetic force of the first solenoid 172a for dischargingelectrode 170, so that the electrode piece 170a moves toward theinsulated wire 13 against the tension of the tension coil spring 176aand stops at a predetermined position. The stopping position at thistime is determined by the setting of the height of the first solenoid172a for the discharging electrode 170. In this connection, the abovefirst solenoid 172a for discharging electrode 170 and the secondsolenoid 172b for discharging electrode 170 are adapted to be adjustablein height, independently. Therefore, it is possible to suitably adjustthe first solenoid 172a for the discharging electrode 170 so that theelectrode piece 170a stops at a position not contacting the insulatedwire 13, for example, at a position a distance of approximately 100 μmin front of the insulated wire 13. Then the second solenoid 172b fordischarging electrode 170 is brought into the off position. Theelectrode piece 170b is pulled toward the insulated wire under thetension of the tension coil spring 176b. At this time, the electrodepiece 170b is positioned relative to the electrode piece 170a by meansof a stopper 177 provided on the swing arm 173b. More particularly, thedistance between the two electrode pieces 170a and 170b depends on theprojected length of the stopper 177. By suitably adjusting the stopper177 to thereby set the distance between the two electrode pieces, forexample, to 200 μm, the insulated wire 13 can be positioned between theelectrode pieces 170a and 170b but not in contact with either of them.To release the wire from the above positioned state, the above describedoperations may be performed in reverse sequence. It is of coursenecessary for properly performing the above operations that theelectromagnetic force of the solenoids 172a and 172b is larger then thetension of the tension coil springs 176a and 176b. Such an effect wasobtained when, for example, by setting the tension of the tension coilspring 176a, 176b to 100 gf when the electromagnetic force of thesolenoid 172a, 172b in a closed state to 500 gf.

With reference to FIG. 10, the insulator pieces 170c and 170d on theside of the opposing surfaces of the electrode pieces 170a and 170b areprojecting a distance 12 from the opposing tip ends of the electrodepieces 170a and 170b. Representing the diameter of the core wire of theinsulated wire 13 by l₃ and the distance between the insulator pieces170c and 170d by ll , the discharge gap length 14 (refer to FIG. 10 andFIG. 19) is set to satisfy

    l.sub.2 <l.sub.4 <l.sub.2 +(l.sub.1 -l.sub.3)/2.

When set so that l₂ =200 μm, l₃ =30 μm, and l₁ =100 μm, the dischargegap can be precisely set as

    200 μm<l.sub.4 <235 μm,

and therefore, a stabilized discharging state can be attained.

Although, in FIG. 9 and FIG. 10, the insulator pieces 170c, 170ddisposed around the electrode pieces 170a and 170b were shown asseparated into upper and lower pieces and bonded to the electrode piece,the invention is not so limited. For example, the insulator pieces 170cand 170d can each be formed into an integrated body and each of theelectrode pieces 170a and 170b can be put into the integral body so thatthe electrode piece 170a and 170b may not easily come off even if it issubjected to repeated impulses during the operation.

Although, with reference to FIG. 9, the invention is not limited to thefirst solenoid 172a for discharging electrode, the second solenoid 172bfor discharging electrode, and tension coil springs 176a, 176b.Alternatively, an actuator such as a linear motor or rotating motor maybe used instead of the solenoid and a spring element such as acompression spring or plate spring may be used instead of the tensioncoil spring.

The circuit configuration of the discharge power circuit 18 to which theelectrode pieces 170a and 170b are connected is described below withreference to FIG. 16. The discharge power circuit 18 includes a powercircuit controlling unit 18d, a high voltage generating unit 18a forproducing a discharge spark S between the insulated wire 13 and thedischarging electrode 17, a low voltage generating unit 18g formeasuring the resistance of the total length of the insulated wire 13, adetecting unit 18b for detecting the wire, a memory unit 18c for storingthe detected value, and resistors R₁ -R₄ for measuring voltage and formeasuring current connected in parallel and series. Between the highvoltage generating unit 18a and the insulated wire 13 and between thelow voltage generating unit 18g and the electrode piece 170b, switches18e and 18f are respectively disposed. More particularly, apredetermined high voltage is applied between the electrode piece 170band the core wire 13a of the insulated wire 13 when the switch 18e isclosed, and a predetermined low voltage from the low voltage generatingunit 18g is applied therebetween when the switch 18f is closed.

The reason for performing control of the discharge voltage by the use ofthe discharge power circuit 18 of the above described configuration isas described below. When an insulated wire 13 is used as in the presentembodiment, differing from the case where a bare wire is used, the totallength of the insulated wire 13 wound around the wire spool 12contributes to a voltage drop ΔV in the discharge circuit. Therefore, ifthe wire length is neglected and a fixed voltage is applied at alltimes, the discharge voltage may vary making it difficult to form astabilized ball 13c. For example, when the core wire 13a of theinsulated wire 13 is made of a gold wire of 30 μm in diameter and thetotal length of the wire wound around the wire spool 12 is 1000 m, thetotal resistance value of the insulated wire 13 right after the wirespool 12 has been mounted becomes approximately 34 kΩ. As thedischarging condition for forming ball 13c of approximately 75 μm indiameter at the wire end of the core wire 13a, a condition of 100 mA ofdischarge current and 0.5 msec of discharging time may be considered.From this, the voltage drop ΔV in the insulated wire 13 immediatelyafter a new wire spool has been mounted becomes as high as 3400 V.

The voltage drop V' across the discharge gap between the electrode piece170b and the wire end 13e can be obtained from the discharge current andthe discharge gap as described later. If the voltage drop is assumed tobe 300 V or so, for example, the voltage V required for producing adischarge spark S becomes the sum of both the voltage drops, i.e.,V=3400+300=3700 V.

The value of the above described voltage drop ΔV gradually decreases asthe bonding work proceeds and the insulated wire 13 is used, andfinally, it becomes virtually 0 V when the insulated wire 13 wound onthe wire spool 12 is used up. Therefore, if a constant voltage isapplied from the beginning of use of the insulated wire 13 of a new wirespool 12 to the end, the produced ball 13c will change greatly in size.

To stabilize the formation of the ball 13c, the discharge power circuit18 is used in the present embodiment for performing control as describedbelow. Immediately after a ball 13c has been made, the switch 18e isopened and the switch 18f is closed so that the low voltage generatingunit 18g is connected to the discharge circuit. In this state, thecapillary 10 is lowered so that the ball 13c and the electrode piece170b is shorted, and a relatively low voltage V4 is supplied from thelow voltage generating unit 18g. At this time, the voltage V₃ across theresistor R₄ is measured by the detecting unit 18b. The resistance R ofthe total length of tile insulated wire 13 is calculated by

    R=R.sub.4 ×[(V.sub.4 /V.sub.3)+1] (Ω)

For example, when V₄ =100 (V) and R₄ =100 (Ω), and if V₃ =0-5 (V) ismeasured, the resistance value of the insulated wire 13 in its totallength is R=20.1 kΩ. When the optimum current in the discharge forforming a ball 13c is set to I_(OPT) =0.1 A, the voltage drop ΔV in theinsulated wire 13 becomes

    ΔV=20100×0.1=2010 (V).

The sum total of this value and total voltage drop V' across thedischarge gap becomes the set voltage V_(OPT) at the time of ballformation, which is expressed as

    V.sub.OPT =ΔV+V'

The above value V_(OPT) is stored in the memory unit 18c and it is usedfor forming a ball 13c next time. More particularly, when the next ballis to be formed, the switch 18f is opened and the switch 18e is closedand the high voltage generating unit 18a is connected to the dischargecircuit. Then, upon issuance of an instruction to start a discharge fromthe control unit 20 to the power circuit controlling unit 18d, the powercircuit controlling unit 18d reads out the value V_(OPT) from the memoryunit 18c and gives an instruction to the high voltage generating unit18a to generate the voltage of that value. The electrode piece 170b andthe wire end 13e are enabled to form a ball 13c in virtually the samecondition as before.

The calculating method of the voltage drop V' across the discharge gapis described as follows.

Generally, the gap voltage depends on such parameters as atmosphere ofdischarge, atmospheric pressure, electrode material on the negativeside, length of discharge gap, and discharge current, of which those tobe specifically taken into consideration are the discharge gap lengthand the discharge current.

In FIG. 20 is shown a graph of gap voltage drops obtained fromexperimental results. From FIG. 20, representing the gap voltage at thetime when the discharge gap length is 0.02 mm by V₀ ' and the variationin the gap voltage when the discharge gap length is changed to 1.0 mm byΔV', it is known, under the condition of a constant discharge current,that the following formula holds true

    V'=270+G×ΔV'(V),

where G represents the discharge gap (mm).

According to FIG. 21, in which the discharge current I is indicatedalong the abscissa in a logarithmic scale a gap voltage drop ΔV' isindicated along the ordinate, the characteristic is represented by astraight line in the logarithmic plot, and the characteristic is givenin a numerical expression as follows.

    ΔV'=280-100 log.sub.10 I (V).

From the above two expressions, the gap voltage V' is expressed, as afunction of the discharge gap length G and the discharge current I, as

    V'=270+G×(280-100 log.sub.10 I) (V), where each constant varies with the initial conditions in the wire bonding apparatus, such as material of the core wire 13a, material of the discharging electrode 17 (electrode piece 170b), and atmosphere of discharge. Therefore, values must be obtained by performing experiments or the like in advance.

Since the above expression is that just obtained by applyinginterpolation on the experimental results, its range of application islimited to a range in which the discharge current is limited within therange of I=7-220 mA and the discharge gap is limited within the range ofG =0.02-1.0 mm.

By having the above described function stored in the memory unit 18c,even if the setting of the discharge current I in a discharge gap ischanged, the suitable gap voltage V' can be calculated from the aboveexpression at all times and the value can be used for the series ofcalculations of the applied voltage. It becomes possible to constantlyperform stabilized discharging for forming the ball 13c and removing theinsulating film 13b along the total length of the insulated wire 13.

The way of controlling reducing the applied voltage as the insulatedwire 13 becomes shorter was described above. However, if the appliedvoltage becomes less than 1000 V or so, it is difficult to start thedischarging following dielectric breakdown. In such a case, it may bepracticed, as shown in FIG. 22, to apply a voltage, 2000-4000 V, forexample, for causing the dielectric breakdown before the main dischargetakes place for such a short period of time as 0.01-0.05 msec. In thisperiod the energy used is negligible as compared with the totaldischarge energy.

A specific example of calculation of the applied voltage is described asfollows with reference to FIGS. 1, 2(a)-2(j) and 16. First, theinsulated wire 13 is passed through the capillary 10 and the firstclamper 14 is closed so that the insulated wire 13 is fixedly held withthe wire 13e sticking out approximately 1 mm from the front end of thecapillary 10. At this time, the wire end 13e may have previously beenheated by a gas burner or the like, not shown, to have the insulatingfilm 13b completely removed from the wire end 13e. What is needed isthat a portion of the core wire 13a is exposed at the position of thewire end 13e facing the discharging electrode surface. Then thecapillary 10 is lowered and the exposed portion of the core wire 13a ofthe wire end 13e is brought into contact with the surface of theelectrode piece 170b. At this time, by providing a short detectioncircuit (not illustrated), within the discharge power circuit 18, thecontact condition between the wire end 13e and the electrode piece 170bmay be detected so that the descent of the capillary 10 is stopped.Then, the switch 18e within the discharge power circuit 18 is opened andthe switch 18f is closed in turn. In this state, a relatively lowvoltage is supplied from the low voltage generating unit 18g to thecharge circuit including the total length of the insulated wire 13. Atthis time, the applied voltage V₄ and a voltage V₃ across the resistorR₄ inserted in series with the circuit are measured by the detectingunit 18b. The coil resistance R of the insulated wire 13 wound aroundthe wire spool 12 is calculated according to

    R=R.sub.4 ×[(V.sub.4 /V.sub.3)+1](Ω)

Here with V₄ =100 (V) and R₄ =100 (Ω) and if V₃ is measured as V₃ =0.5(V), the result is obtained as R=20.1 (kΩ).

During the above described processes, measurement of the coil resistanceof the insulated wire 13 at the time when the wire spool 12 has beenmounted is completed. The resultant value of the coil resistance R isstored in the memory unit l8c within the discharge power circuit 18.

The discharging for forming a ball as the first discharging will bedescribed with reference to FIG. 17. As an example, when a ball 13c of75 μm in diameter is formed at the wire end 13e of the insulated wire 13whose core wire 13a is made of a gold wire of 30 μm in diameter, theelectrode piece 170b for ball formation is set to the negative polarity,the discharging time is set to 0.5 msec, the discharge current is set to0.1 A (100 mA), and the discharge gap is set to approximately 0.5 mm.The applied voltage to achieve the discharge current is as describedabove, i.e., the desired current I=0.1 A, is calculated as describedbelow. First, the coil resistance R calculated as describe above and theabove set desired current I, the voltage drop ΔVa in the coil portion ofthe insulated wire 13 becomes ##EQU1## Then, the voltage drop Va' acrossthe discharge gap is calculated from G=0.5 mm and I=100 mA as ##EQU2##The applied voltage V under the above described conditions becomes##EQU3## The thus obtained applied voltage V is stored in the memoryunit 18c. In performing the ball forming discharging, the switch 18f isopened after the switch 18e has been closed and the high voltagegenerating unit 18a is activated.

The power circuit control unit 18d reads out the above described appliedvoltage V (=2320 (V)) stored in the memory unit 18c and issues aninstruction to the high voltage generating unit 18a to generate thevoltage of the value. It is possible to supply the discharge currentI=100 mA as the desired current to the electrode piece 170b as thedischarging terminal for ball formation. The relationship between theapplied voltage V and the voltage drop ΔVa is illustrated in FIG. 18.

The above description has been of the discharge conditions for ballformation for the first bonding. Below is described discharge conditionsfor removing the insulating film 13b for the second bonding withreference to FIG. 19.

Where an insulated wire 13 is formed, for example, of a core wire 13a ofgold of 30 μm in diameter and an insulating film 13b made of heatresisting polyurethane and coated over the core wire to a thickness of Iμm is used and the insulating film 13b is to be removed by thermaldecomposition at the portion to be bonded in the second bonding, whichextends over a range of 500 μm along the length of the core wire is bedescribed as follows. The discharge conditions are with the dischargingelectrode 17 set to the negative polarity, the discharge time set to 10msec, the discharge current (desired current Ib) set to 0.01 A (10 mA),and the discharge gap length set to 0.2 mm.

The applied voltage for obtaining the desired current Ib=10 mA iscalculated in the following way. First, from the coil resistance value Rof the insulated wire 13 calculated in the initial setting as describedabove and the desired current value Ib as set above, the voltage dropΔVb in the coil portion is calculated as ##EQU4## Then, the voltage dropVb' across the discharge gap (G=0.2 mm) becomes ##EQU5## From this, thedesired value of the applied voltage V is obtained as ##EQU6## Theobtained voltage value (V=505 (V)) is stored in the memory unit l8c inthe discharge power circuit 18.

In actually removing the insulating film 13b by discharging, the switch18e is closed and the high voltage generating unit 18a is renderedoperative, and the applied voltage (V=505(V)) stored in the memory unit18c as described above is read out according to an instruction from thepower circuit controlling unit 18d. Thereafter, an instruction is givento the high voltage generating unit 18a to generate the voltage of thisvalue. In this way, it is made possible to pass the desired dischargecurrent I=10 mA through the two electrode pieces 170a and 170b as thedischarging terminals for removing the insulating film.

When the applied voltage calculated as described above is lower than1000V, it frequently becomes difficult to start a discharge in astabilized manner because of too low a voltage. Therefore, as an initialvoltage to start the discharge, a high voltage of, for example, 2000V orso may be applied, as shown in FIG. 22, for such a short period of timeas 0.01 msec that will not affect the entire discharge. When the appliedvoltage obtained by calculation is as high as above 2000V, it is ofcourse unnecessary to specially apply such an initial voltage.

The above described sequence of processes is performed based on the coilresistance R (R=20.1 (kΩ) in the above example) calculated immediatelyafter a wire spool 12 has been loaded. Since the length of the insulatedwire 13 gradually decreases as the bonding process is advanced, thedecrease in the coil resistance R with the decrease in the total lengthof the insulated wire 13 must be considered in the above describedcalculation of the applied voltage V. The measurement of the resistanceof the wire in a coil having a length gradually decreasing in thedescribed way should be performed in the same way as in the abovedescribed initial setting. More particularly, for each wire bondingprocess, the ball 13c formed at the wire end 13e of the insulated wire13 is put into contact with the discharging surface of the electrodepiece 170b and the switch 18e within the discharge power circuit 18 isopened and the switch 18f is closed in turn. In this state, a relativelylow voltage is supplied from the low voltage generating unit 18g to thedischarge circuit including the total length of the insulated wire 13.Thereupon, the applied voltage V₄ and the voltage V₃ across the resistorR₄ connected in series with the circuit are measured in the detectingunit 18b. In this way, the coil resistance R of the insulated wire 13wound around the wire spool 12 can be measured once for each bondingcycle in the same manner as in the above described initial setting.

Although the coil resistance R is measured once for each bonding cycleas described above, such measurement may be performed intermittently,i.e., once for each several cycles of bonding or for bonding of eachunit of semiconductor chip 3, provided that the coil resistance R doesnot change appreciably during such an interval.

Calculation of the applied voltage V may also be performed in thefollowing manner. It is presupposed that the initial setting has alreadybeen made and the coil resistance value R has already been determinedand the bonding process is being sequentially advanced. First, it isassumed that the switch 18e in the discharge power circuit 18 is closed,the switch 18f is opened, and the high voltage generating unit 18a isoperative. In this state, a discharge voltage for ball formation issupplied from the high voltage generating unit 18a so that a current ispassed through the circuit. Then, a voltage V₁ across a resistance R₃connected in parallel with the circuit for voltage detection 18b and avoltage V₂ across a resistor R₁ connected in series with the circuit forcurrent detection are measured. The generated voltage V and the currentI which flows can be calculated from these voltages V₁, V₂ as follows.

    V=(R.sub.2 +R.sub.3)/R.sub.3 ×V.sub.1,

    I=V.sub.2 /R.sub.1.

If the voltage drop across the discharge gap is represented by Va', thevalue of the coil resistance R of the insulated wire 13 is obtained from

    R=(V-Va')/I.

If resistance values are set so that R₁ =10ΩR₂ =10 MΩ, and R₃ =100 kΩand if the results of the measurements are V₁ =2(V) and V₂ =1(V), then##EQU7## By using the value I=0.1A (=100 mA) and the value of thedischarge gap G (for example, G=0.5 mm), ##EQU8## From these values, thecoil resistance is obtained as ##EQU9## The value R of the coilresistance at this time is stored in the memory unit 18c the same asbefore. Based on the thus obtained value of the coil resistance R, thesuitable value of the applied voltage for discharging for forming a ball13c in the next cycle can be obtained.

Although the above described measurement was that performed at the timeof controlling the ball forming discharging at the first bondingposition of the insulated wire 13, a similar measurement may be made atthe time of control at the second bonding position, i.e., at the time ofdischarging for removing the insulating film. By virtue of the abovedescribed control performed in the discharge power circuit 18, itbecomes possible, at any desired time, to accurately detect the changein the coil resistance R with decrease in the length of the insulatedwire 13 the resulting from the bonding process. Therefore, it is madepossible to set up a suitable discharge voltage and formation of theball 13c of a fixed form and removal of the insulating film 13bextending over a fixed range can be performed at all times along thetotal length of the insulated wire 13 to achieve stabilized bonding.

The bonding process with the above described techniques applied theretowill be described below chiefly referring to FIG. 2.

In FIG. 2(a), the first clamper 14 is in a closed state right above thesemiconductor chip 3 of the bonding stage 2 with the insulated wire 13in a clamped state and the wire end 13e of the insulated wire 13 is in astate projecting out of the front end of the capillary 10 a length La(for example, La=0.5 mm-1.0 mm). At this time, the wire end 13e has itsinsulating film 13b removed 0.1-0.4 mm by means of the above describeddischarging technique. This process will be given later in connectionwith the process of FIG. 2(i).

In FIG. 2(b), the electrode piece 170b of the discharging electrode 17(FIG. 1) is brought right under the wire end 13e of the insulated wire13 facing the wire end 13e across a gap (discharge gap) of apredetermined length. A ball 13c is formed by virtue of the abovedescribed application of a high voltage from the discharge power circuit18. The discharge gap at this time may be set to a length ranging from0.2 mm to 1.0 mm or, preferably, to a length ranging from 0.3 mm to 0.7mm. Although not illustrated, the ball 13c immediately after beingformed may be put into contact with the discharging surface of theelectrode piece 170b (FIG. 16) and the coil resistance R of theinsulated wire 13 may be measured as described above.

In FIG. 2(c), the second chamber 15 is closed and frictionally clampedthe wire, while the first clamper 14 is in an opened state. Infrictional clamping 1.0 gf-4.0 gf of tension is applied to the insulatedwire 13 as described above. In this state, the capillary 10 is loweredtoward the first bonding position (the first position) on thesemiconductor chip 3 as shown in FIG. 2(d), and then, the ball 13c iscaught by the front end of the capillary 10. The entire insulated wire13 is lowered.

FIG. 2(e) illustrates the first bonding operation. A load of 50-100 gfis imposed on the capillary 10 with the ball 13c held at the front endof the capillary 10 put in abutment with the semiconductor chip 3. Inaddition, ultrasonic vibration, for example, at 60 kHz and of anamplitude of 0.5 μm-1.0 μm is applied from the ultrasonic vibrator 11 tothe capillary 10. The ball 13c is subjected to a multiple effect ofultrasonic vibration and heat from a heater 2a within the bonding stage2 to heat it up to 200° C. The insulated wire 13 is bonded to thesemiconductor chip 3 (bonding pad 3b) within a bonding time of 20-40msec. Such a bond is attained by mutual diffusion of atoms of gold (Au)forming the ball 13c and aluminum (Al) forming the bonding pad 4aaccelerated by the heat and ultrasonic vibration. At this time, thesecond clamper 15 is in an opened state (unclamped) so that theinsulated wire 13 is not restricted for movement by the first clamper 14or by the second clamper 15.

FIG. 2(f) illustrates the capillary 10 which is elevated a predetermineddistance after the above described first bonding has been completed. Atthis time, the amount of elevation of the capillary 10 is controlledsuch that the front end of the capillary 10 is stopped a distance fromthe end of the insulated wire 13 where the insulating film 13b is to beremoved (the exposed portion 13d). The capillary 10 has reached thehighest position, as illustrated but this position is not limitative. Itmay also be arranged such that the exposed portion 13d is positioned atthe front end of the capillary 10 while it is in the descending coursetoward the bonding position (the inner lead 4b).

From the position illustrated in FIG. 2(f), the capillary 10 may beelevated to a predetermined height and then lowered toward the secondbonding position (the inner lead 4b) and the second clamper 15 may bebrought into the frictionally clamp state (the state where the firstclamp load is imposed) at a suitable point in the descending course ofthe capillary 10. Then, an upwardly pulling force acts on the insulatedwire 13 against the descending capillary 10 so that occurrence ofinsufficient pulling of the wire is prevented and the height of the wireloop can be controlled to be stable. By bringing the second clamper 15into the frictional clamped state while the capillary 10 descends,occurrence of an abnormal loop due to insufficient pulling in of thewire into the capillary is prevented. The necessary length of the wireto form the wire loop can be controlled to be uniform and the positionof the wire to be bonded to the second bonding position (the positionwhere the insulating film is to be removed) can be set with highaccuracy.

FIG. 2(g) illustrates the capillary 10 landed on the second bondingposition (the inner lead 4b) and the second bonding being performed. Themovement of the capillary 10 in the horizontal direction on the drawingis provided by a relative movement of the X-Y table 5 of FIG. 1. Thebonding load on the capillary 10 of 100-150 gf, bonding time of 10-30msec, the ultrasonic frequency at 60 kHz and amplitude of 1.0-2.0 μm,and the bonding temperature at around 200° C. are preferred as bondingconditions in the above described second bonding. Under such conditions,mutual diffusion of Au atoms at the exposed portion 13d of the core wire13a of the insulated wire 13 and Ag atoms in the silver coating on theinner lead 4b are accelerated and the bonding is thereby achieved.

In the present embodiment, since, at this time, an exposed portion 13dwhere the insulating film 13b is removed has been formed in advance atthe second portion to be bonded of the insulation wire 13 and ultrasonicvibration is applied to the area of the peripheral surface of the corewire 13a and the inner lead 4b held in direct contact with each other,the following benefits are obtained. First, the need for application ofthe ultrasonic vibration for removing the insulating film 13b can beeliminated. More particularly, application of multiple stages ofultrasonic vibration required for mechanically breaking and removing theinsulating film 13b at the second portion to be bonded is eliminated sothat bonding can be performed effectively. Second, the bonding strengthin the second bond can be maintained at a high strength. Moreparticularly, since the insulating film 13b has already been removed andthe core wire 13a is exposed when the second bonding is performed, suchthings as pieces of the insulating film do not get in the way at thetime of application of the ultrasonic vibration. Hence, high bondingstrength can be obtained and reliability on the bond can be enhanced.Third, since the insulating film 13b has been removed for a suitabledistance of where the second bonding is performed, bonding at lowtemperature can be attained, and the temperature at the time of thesecond bonding can be made virtually the same as in bonding a bare wire.Therefore, damage of the device due to heat, or fatigue can beprevented. Further, since the insulating film 13b is removed bydischarging, the film thickness of the insulating film 13b can be madelarger and hence the insulating capability of the insulated wire 13 canbe maintained high.

FIG. 2(h) illustrates the capillary 10 after the completion of thesecond bonding elevated by a distance L₁ from the surface of the leadframe 4 without moving the X-Y table 5. The distance L₁, at this time iscalculated in the later described method dependent on information on thefirst and second bonding positions, conditions in the initial setting ofthe apparatus, and so on. When the capillary 10 has been raised to theheight of L₁, the first clamper 14 is closed and the insulated wire 13is put into a clamped state.

FIG. 2(i) shows a state of the capillary 10 further elevated to theheight of L₂ with the first clamper 14 closed and thereby the insulatedwire 13 is clamped. At this time, the first clamper 14 with theinsulated wire clamped therein ascends in association with the capillary10 and the insulated wire 13 is broken at the position of the secondbonding. As a result, the insulated wire 13 projects out of the frontend of the capillary 10 by the length L₁.

Then, as illustrated both the electrode pieces 170a and 170b of thedischarging electrode 17 surround the insulated wire 13 from two sidesin a non-contact manner and a voltage controlled as described above issupplied from the discharge power circuit 18 to the dischargingelectrode 17. Discharges are produced between the core wire 13a and theelectrode pieces 170a and 170b through the insulating film 13b. Thedischarge energy at this time removed a portion of the insulating film13b at a predetermined position on the insulated wire 13. As thedischarge conditions, the side of the discharging electrode 17 is givena negative polarity relative to the insulated wire 13, the dischargetime is set to 2-20 msec, the discharge current is set to 5-30 mA, andthe discharge gap is set to 0.1-0.5 mm. As described above, however, itis preferable to set the discharge time to 10 msec, the dischargecurrent (the desired current Ib) to 0.01A (10 mA), and the discharge gaplength to 0.2 mm. These values, however, are not limiting. What isessential is that a suitable length of the insulating film 13b is heatedto its thermally decomposed temperature, approximately 600° C., withoutheating the wire up to the melting point of gold (AU), 1063° C. In thisconnection, if the discharge conditions are set as described above, itbecomes possible to thermally decompose and completely remove theinsulating film 13b without forming any ball by the discharge at theportion of the insulated wire 13 on which insulation is being removed.The range of removed area of the insulating film can be controlled to be0.1 mm-1.0 mm, or further, to be 0.4 mm-0.6 mm, steadily.

If the set height of the electrode surface of the discharging electrode17 at this time is represented by L₃ and the distance from the front endof the insulated wire 13 to the center, of the removed area isrepresented by L₄, the following geometrical relationship holds betweenthe above described heights. L₁ and L₂

    L.sub.2 -L.sub.3 =L.sub.1 -L.sub.4.

Therefore,

    L.sub.1 =L.sub.2 -L.sub.3 +L.sub.4,

where, L₂ and L₃ are values setting and L₄ is a value to be determinedby calculation based on information between positions of bonding in thenext bonding operation.

After the above described discharging, the discharging electrode 17retreats away from the lower end of the capillary 10 and the secondclamper 15 is closed. At this time, the second clamper 15 is put intothe fixed clamp state, where the second clamp load of 50-150 gf is setand the insulated wire 13 is completely restricted in its movement.

As illustrated in FIG. 2(j), the first clamper 14 is opened and thecapillary 10 is lowered by a distance L₈ with respect to FIG. 2(i).Since the insulated wire 13 is restricted in its movement by the secondclamper 15 at this time, the insulated wire 13 is pulled into theinterior of the capillary 10 by the distance L₈, and hence, the frontend of the insulated wire 13 is left projecting out of the front end ofthe capillary 10 by a length Lg. The first clamper 14 is closed and thesecond clamper 15 is opened in turn. Then the capillary 10 is elevatedto the initial height L₂ and the X-Y table 5 makes a predeterminedamount movement and the initialization for the next bonding cycle is setup (refer to FIG. 2(a)).

The above described heights of the capillary (bonding tool) 10, theoperating timing of mechanisms for on/off setting of the solenoids forthe first clamper 14 and second clamper 15, etc. in FIG. 2(a) FIG. 2(j)are illustrated in FIG. 3.

Among the values L₀, L₁, L₈, L₉, and L₁₀ described in FIG. 2(a)-FIG.2(j), the relationships hold as follows:

    L.sub.1 =L.sub.8 +L.sub.9,

    L.sub.2 =L.sub.0 +L.sub.10 +L.sub.9.

The height L₀ of the electrode surface of the discharging electrode 17for forming the ball is determined in the initial setting of theapparatus. The discharge gap L₁₀ and the tail length L₉ are determinedby the initial setting made by the operator. The initial height L₂ islogically determined from these value. At this time, the distance L₁₀can be set with high accuracy, for example, by bringing the front end ofthe capillary 10 into contact with the discharging surface of theelectrode piece 170b of the discharging electrode 17 and detecting theheight by the use of a position detecting mechanism, not shown, withinthe bonding head 6. The height L₃ of the discharge electrode forremoving the insulating film can also be calculated with ease.

The distance L₄ can be calculated according to FIG. 4 from the followingexpression

    L.sub.4 =L.sub.6 +L.sub.7

L₆ is the length of the insulated wire 13 required for the next bondingand L₇ is the length necessary for forming a ball 13c. The length L₇ isobtained based on the diameter d of the core wire 13a of the insulatedwire 13 and the diameter D (μm) of the ball 13c (refer to FIG. 2(b))from

    L.sub.7 =(2/3)×(D.sup.3 /d.sup.2).

By setting, for example, d=30 μm, L₇ becomes

    L.sub.7 =7.41×10.sup.-4 ×D.sup.3 (μm).

Assuming now that the accuracy of the diameter D of the ball 13c to be75 μm±5 μm, it is known that the length L₇ is reproduced at the accuracyon the order of 320±50 μm. The above expression is indicated as a graphin FIG. 6.

The length L₆ required for the bonding is obtained approximately fromthe wiring distance L₁₅ obtained upon detection of the bonding distanceand the loop height L₁₄, i.e.,

    L.sub.6 =L.sub.14 +L.sub.15.

The loop height L₁₄ is a value determined by wiring conditions such asmechanical property of the core wire 13a depending on its manufacturingmethod, the difference in level of the first and second bondingpositions, the locus of the capillary 10, the amount of back tension atthe time of wiring, and the discharge condition for ball formation,which is affected in particular by the wiring distance L₁₅. Therelationship between the loop height L₁₄ and the wiring distance L₁₅ isexperimentally obtained in advance and it is stored in the control unit20. Therefore, a loop height L₁₄ corresponding to a wiring distance L₁₅can be obtained.

As an example of experiments without limitation of the invention, anempirical relationship between the loop height L₁₄ and the wiringdistance L₁₅ under a specific condition is illustrated in FIG. 5.According to FIG. 5, L₁₄ can be expressed as a linear function of L₁₅,i.e.,

    L.sub.14 =0.05×L.sub.15 +0.15 (mm).

The value L₁₄ may of course be a function of multiple order of L₁₅.

By the use of the above expressions, the length L₄ from the front end ofthe insulated wire 13 to the position where the insulating film isremoved (the exposed portion 13d) to be used for the next bonding can beobtained by calculation. Further, from the above values, L₁, L₂, and L₈can be calculated, and by using these values, the bonding in an optimumpositional relationship can be achieved.

Although it was not mentioned above for simplicity of explanation, thefollowing things with reference to FIG. 7 must be considered inperforming sequences of bonding operations. First, bonding positions aredetected (step 701) and the calculation of the wiring distance L₁₅ isperformed (702). Based on the obtained information, the removed positionL₄ of the insulating film 13b in each bonding cycle is calculated (703).At (704) it is determined if the first wire is being processed. Dummybonding is performed onto the tab 4a if the answer is "yes" at (704).The wiring distance in the dummy bonding is set to a fixed valueaccording to an instruction from the control unit 20. After the dummybonding has been performed, the real bonding is performed (706).Thereafter, the insulating film 13b is removed at the second portion tobe bonded, i.e., at the position corresponding to the value L₄ that hasalready been calculated (708). Then the processes after step 704 arerepeated. If the bonded insulated wire 13 has been the last wire in thepresent semiconductor chip 3 (707), the next portion of the wire issubjected, as the dummy wire, to removal of the insulating film 13b atthe position to be bonded at the fixed distance as indicated by theinstruction from the control unit 20 (709). Subsequently, the nextsemiconductor chip 3 (the lead frame 4) is set up on the X-Y table 5(710) such that the above described steps 701-704 are repeated. Sincethe bonding at this time is that of the first wire for the newsemiconductor chip 3, dummy bonding at step 705 is performed first andthe dummy wire prepared as described above is used for the dummybonding. At this time, since the wiring distance in the dummy wire is ofa fixed length at all times, it is assured that the bonding from thefollowing first wire can be steadily performed with an optimum wiringlength. Since the bonded condition in the dummy bonding does notdirectly affect the reliability of the product, after the last wire hasbeen bonded to the preceding semiconductor chip 3, it does not have tobe bonded to the position corresponding to the wiring distance of thefirst bonding in the new chip.

Referring to FIG. 4, the exposed length L₁₃ of the core wire 13a at thesecond bonding position can be controlled to be L₁₃ =0.1-0.4 mm bylimiting, for example, fluctuation of the range of removal to 0.4-0.6mm, fluctuation due to the calculation error of the length L₄ caused byfluctuation of the loop height (for the same wiring distance) to ±50 μm,and fluctuation due to the calculation error of the length L₇ caused byfluctuation of the ball diameter to ±50 μm. Therefore, shorting betweenwires or the like can be effectively prevented and highly reliablesecond bonding can be achieved.

While the invention has been described specifically, it is apparent thatthe present invention is not limited to that described in the abovepreferred embodiment but various modifications of the invention arepossible without departing from the spirit and the scope of it.

Although the invention has been described in the foregoing as to theapplication where it is applied to a preferred field of utilization,i.e., the wire bonding technique with an insulated wire using a goldwire as the core wire, the applications of the invention are not limitedthereto. For example, the invention can also be applied to the wirebonding process with an insulated wire using other conducting metalssuch as a copper wire or an aluminum wire as the core wire.

Benefits obtained from representative aspects of the invention describedin its preferred form are briefly described as follows. Formation of aball and exposure of the core wire at the first portion to be bonded andthe second portion to be bonded of an insulated wire can be achievedwithout making the structure of the apparatus complex and wire bondingusing an insulated wire and providing high bonding strength can beperformed. The wire can be constantly kept in a fixed degree ofslackening without fluctuation in the tension of the wire above thebonding tool. Therefore, a constantly stabilized bonding operation canbe performed.

Since an optimum applied voltage adjusted for decrease in length of theinsulated wire can be constantly supplied, formation of a ball of astabilized size and removal of the insulating film in a stabilizedlength can be achieved. Highly reliable and stabilized wire bonding canbe performed irrespective of the length of the wire wound around thewire spool. Since the wire loop can be controlled to a steady height,bonding of the portion of the wire where the insulating film is removedto the object can be performed with high accuracy. Stabilized bondingwith optimum wiring length can be performed from the first wire for eachsemiconductor chip at all times. It is possible to improve reliabilityof wire bonding of the semiconductor devices fabricated through the wirebonding process having benefits as described above.

Embodiment 2

FIG. 23(a)-FIG. 23(j) are process drawings showing an example ofoperations in an embodiment of a method for wire bonding of the presentinvention and FIG. 24 is a side view schematically illustratingstructure of an embodiment of an apparatus for wire bonding of thepresent invention. Referring first to FIG. 24, the wire bondingapparatus is described. On a bed plate 301, a bonding stage 302 isarranged such that its longitudinal direction is in the directionperpendicular to the page. On the bonding stage 302, there is mounted alead frame 304, which comprises a train of a plurality of tabs 304adisposed in the center of the lead frame 304 in the directionperpendicular to the page at a predetermined pitch. Each tab 304a has asemiconductor chip 303 mounted thereon and a plurality of leads 304bsurrounding the semiconductor chip 303 mounted on individual tabs 304a.The bonding stage 302 incorporates a heater 302a, used for heating thelead frame 304 mounted thereon together with the semiconductor chips 303to a predetermined temperature. Above the bed plate 301 and at a side ofthe bonding stage 302, there is arranged an X-Y table 305 movable on ahorizontal plane. On the X-Y table 305, there is supported a bondinghead 306 by a shaft 307 for swinging in a vertical plane, with one endpositioned above the bonding stage 302. The other end of the bondinghead 306 is connected with a linear motor 308 mounted on the X-Y table305 for producing vertical movement. On the end of the bonding head 306at the side above the bonding stage 302, there is supported horizontallya bonding arm 309. At its front end located directly above the bondingstage 302, there is fixedly held a capillary 310 with a wire insertionhole (not illustrated) axially passing therethrough held in the verticaldirection. At the side of the base of the bonding arm 309, there isprovided an ultrasonic vibrator 311, for applying predeterminedultrasonic vibration to the capillary 310 fixed to the front end of thebonding arm 309 at any time. Into the wire insertion hole of thecapillary is inserted an insulated wire 313 fed from a reel 312. Theinsulated wire 313 is formed of a core wire 313a made of a conductivemetallic material (for example, Au) of, for example, 25-30 μm indiameter and a insulating film 313b made of a high polymer resinmaterial having an insulating property (for example, polyurethane orheat resisting polyurethane) of I-2 μm in thickness covering the corewire. In the path of the insulated wire 313 between the capillary 310and the reel 312, there is provided, fixed to the bonding head 306, afirst wire clamper 314, which when necessary prevents the insulated wire313 from being delivered from the capillary 310 by restricting theinsulated wire 313 in its movement, and which provides vertical movementtogether with the capillary 310 by swinging movement of the bonding head306 by means of the linear motor 308. In the path of the insulated wire313 between the capillary 310 and the reel 312, there is furtherprovided a second wire clamper 315 fixed to the side of the bed plate301 independently of the bonding head 306, and the second wire clamper315 operates independently of the first wire clamper 314 and preventsthe insulated wire 313 from being delivered from the capillary 313 whennecessary. Further, in the path of the insulated wire 313 between thecapillary 310 and the reel 312, there is provided a back tensionmechanism, which constantly applies a predetermined value of tensionpulling back the insulated wire 313 in the direction from the capillary310 to the reel 312 by blowing an air stream at a predetermined flowvelocity to the insulated wire 313 at its periphery. The insulated wire313 moves in the direction to return to the reel 312 when both the firstwire clamper 314 and the second wire clamper 315 are opened. In thevicinity of the capillary 310 fixed at the front end of the bonding arm309, there is provided a first discharging electrode 316 positioned at apredetermined height from the bonding stage 302 for horizontal movementin the direction to come right under the front end of the capillary 310.A predetermined value of discharge voltage from a discharge powercircuit 318 is applied between the same and the insulated wire 313 atpredetermined timing. The first discharging electrode 316 moves whennecessary to the position directly under the front end of the insulatedwire 313 projecting out of the front end of the capillary 310 apredetermined length in such a way that there is left a predeterminedgap between the capillary and the front end of the insulated wire 313. Adischarge is produced between the first discharging electrode 316 andthe front end of the insulated wire 313 to melt the core wire 313a ofthe insulated wire 313 to form a ball 313c by virtue of surface tension.In the vicinity of the first discharging electrode 316 producing thedischarge for forming the ball 313c at the front end of the insulatedwire 313, there is disposed a second discharging electrode 17 formovement in the direction crossing the path of the insulated wire 313drawn out of the front end of the capillary 310 and connected with thedischarge power source 18. The second discharging electrode 317 facesthe insulated wire 313 drawn out of the capillary 310 from both itssides with a desired gap therebetween. Then, discharges are producedbetween the electrode and the core wire 313a through the insulating film313b so that the insulating film 313b covering the core wire 313a isvaporized (thermally decomposed) and removed by energy of the dischargea predetermined length in the longitudinal direction. The process forforming an exposed portion 313d, where the core wire 313a is exposed tothe outside at a predetermined distance from the end of the insulatedwire 313, is performed. The bonding head 306 is provided with an imagerecognition mechanism 319 for picking up images of the semiconductorchip 303 located right under the capillary 310 and the lead frame 304having the semiconductor chip 303 mounted thereon to recognize thepositions of a plurality of later described bonding heads 303a providedon the semiconductor chip 303, a plurality of leads 304b surrounding thesemiconductor pellet 303, etc. The X-Y table 305 on which the imagerecognition mechanism 319 and the bonding head 306 are mounted, thelinear motor 308 for controlling the vertical movement of the bondinghead 306, the first wire clamper 314, the second wire clamper 315, thedischarge power circuit 318, etc. are controlled by the control unit 320so that later described bonding operations are performed through theirassociated operations.

An example of a wire bonding method in which the wire bonding apparatusof the above described structure is employed is described below withreference to FIG. 23(a) to direction perpendicular to FIG. 24 by a feedmechanism provided on the bonding stage 302. A semiconductor chip 303mounted thereon is positioned directly under the bonding head 306 and atthe same time, the lead frame is heated up to the temperature of about200° C. by the heater 302a. A control unit 320 recognizes the distancebetween the bonding pad 303a of the semiconductor chip 303 being bondedand the bonding position of the corresponding lead 304b by means of theimage recognition mechanism 319. At this time, the first wire clamper314 and the second wire clamper 315 on the side of the bonding head 306are both opened and the ball 313a formed at the front end of theinsulated wire 313 is held by the front end of the capillary 310 byvirtue of the tension produced by the back tension mechanism (FIG.23(a)). The X-Y table 5 is suitably moved so that the capillary 310 ispositioned right above the aimed one of the plural bonding pads 303a onthe semiconductor pellet 303. At the same time, the linear motor 308 isoperated so that the capillary 310 is lowered onto the target bondingpad 303a. Ultrasonic vibration at approximately 60 kHz from theultrasonic vibrator 311 is applied to the capillary 310 it presses theball 313c against the bonding pad 303a with a load of 50-150 gf. Theball 313c is compression-bonded to the bonding pad 303a (FIG. 23(b)).Then, the linear motor 308 is operated to thereby elevate the capillary310. The capillary 310 delivers the insulated wire 313 while it ascendsand brings its front end to the same height as the position of theexposed portion 313d of the core wire 313a previously formed at acertain distance from the end of the insulated wire 313 in the manner aslater described (FIG. 23(c)). Further, the X-Y table 305 is driven sothat the capillary 310 is positioned right above one of the plural leads304b. At the same time, the linear motor 308 is driven and the capillary310 is lowered. The portion of the peripheral surface of the core wire313a exposed to the outside at the exposed portion 313d of the insulatedwire 313 is brought into direct contact with the surface of the lead304b and pressed against the surface. Ultrasonic vibration is applied tothe wire so that the peripheral portion of the core wire 313a isthermal--compression-bonded to the surface of the lead 304b (FIG.23(d)). Since, the bonding is performed with the core wire 313a and thelead 304b held in direct contact without the insulating film 313binterposed therebetween, the insulating film 313b or foreign materialsproduced from the insulating film 313b thermally changed in quality donot come between the core wire 313a and the lead 304b or between thecore wire 313a and the capillary 310. Compared with the bonding in theprior art performed through the insulating film 313b, larger bondingstrength and lower electric resistance are obtained at the bondedportion, and thus, a good bonding characteristic is obtained at thebonded portion between the peripheral surface of the core wire 313a ofthe insulated wire 313 and the lead 304b. Further, trouble consequentfrom foreign materials coming into the wire insertion hole does notoccur and the operations for delivering and pulling in the insulatedwire 313 are smoothly performed at all times. Stabilized bonding workcan be continued over a long period of time. The capillary 310 ascends adistance L₁ predetermined by calculation from the surface of the lead304b and stops (FIG. 23(e)). During the course from FIG. 23(a) to FIG.23(e), the first wire clamper 314 and the second wire clamper 315remains open. The first wire clamper 314 is closed and with theinsulated wire 313 clamped the capillary 310 further ascends from theheight L₁ to the height of L₂ and stops. At this time, the insulatedwire 313 is broken in the vicinity of the bonded position on the lead304b. The electric connection by wiring the insulated wire 313 betweenone bonding pad 303a on the semiconductor chip 303 and one correspondinglead 304b of the lead frame 304 is completed (FIG. 23(f)).

Then, the second discharging electrode 317 located at a predeterminedheight L₃ from the surface of the lead 304b approaches the correspondingportion of the insulated wire 313, which is pulled out of the front endof the capillary 310, from both its sides and produces dischargesbetween the electrode and the core wire 313a through the insulating film313b (FIG. 23(g)). By energy of the discharge provided at this time,completely removes the insulating film 313b at the desired distance L₄from the end of the insulated wire 313 is completely removed by thermaldecomposition and the like extending over a predetermined length L₅ inthe axial direction. The exposed portion 313d where the core wire 313ais exposed to the outside is formed. As discharging conditions at thistime, it is preferred to make the polarity of the second dischargingelectrode 317 negative relative to the core wire 313a and set thedielectric breakdown voltage of the insulating film 313b and the gap to2000-4000 V, the discharge current to 10-20 mA, the discharging periodof time to 2-10 msec, and the discharge gap to 0.10.3 mm. Under suchconditions, the insulating film 313b can be steadily removed extendingover a range of 0.4-0.6 mm in the axial direction. By making the lengthL₄ from the front end of the insulated wire 313 to the exposed portion313d equal to the sum total of a length L₆ of the insulated wire 313required for forming a wire loop in the next wiring step and a length L₇required for forming a ball 313c, it becomes possible to bring thecenter of the exposed portion 313d in concurrence with the bondingposition on the lead 304b in the next bonding operation. Among thelengths L₁ -L₇, there are geometrically relationships as follows:

    L.sub.4 =L.sub.6 +L

    L.sub.1 =L.sub.4 +L.sub.2 -L.sub.3

    L.sub.1 =L.sub.6 ++L.sub.7 +L.sub.2 -L.sub.3

The length L₆ of the insulated wire 313 required for forming the wireloop in the next wiring step is determined by the distance between thebonding pad 303a of the semiconductor chip 303 and the bonding positionon the lead 304b to be interconnected and the height of the wire loop.The length L₇ used for forming the ball 313c is determined by thediameter of the core wire 313a and the diameter of the ball 313c. Thelengths L₂ and L₃ are determined by initial setting of the wire bondingapparatus, and therefore, if these factors are determined, the height L₁of the capillary 310 in the step of FIG. 23(e) can be obtained bycalculation. The control unit 320 obtains the height L₁ of the capillary310 by calculation in the step of FIG. 23(e) and brings the height ofthe capillary 310 to L₁ by suitably controlling the operation of thelinear motor 308. First after the formation of the exposed portion 313dby the discharges in the step shown in FIG. 23(g), the second wireclamper 315 closes to restrict the insulated wire 313 in its movementand then the first wire clamper 314 opens. The capillary 310 descends adistance L₈ and stops at the position where the front end of theinsulated wire 313 sticks out of the front end of the capillary 310 by alength Lg (FIG. 23(11)).

From a geometrical relationship among the lengths L₈, L₉ and L₁, it isobtained:

    L.sub.8 =L.sub.1 -L.sub.9.

The length L₉ is that required for forming the ball 313c at the frontend of the insulated wire 313 in the next step and its suitable lengthis 0.5-1.0 mm in the present embodiment. Since the length L₁ is obtainedwhen the earlier described various factors are determined, the L₈ canalso be obtained by calculation. The control unit 320 sets up the lengthL₈ by suitably controlling the linear motor 308. First, the first wireclamper 314 is closed to restrict the insulated wire 313 in itsmovement, the second wire clamper 315 is opened, and the capillary 310ascends to the height L₂ from the surface of the lead 304b and stops. Atthe same time, the first discharging electrode 316 enters the spaceright below the capillary 310 such that it confronts the front end ofthe insulated wire 313 projecting out of the front end of the capillary310 across a predetermined discharge gap L₁₀ (FIG. 23(i)).

A discharge is produced between the front end of the insulated wire 313and the first discharging electrode 316. Then, the insulating film 313bat the end portion of the insulated wire 313 is removed by thermaldecomposition caused by energy of the discharge. At the same time, theend portion of the core wire 313a is melted and a ball 313c is formed bythe surface tension of itself (FIG. 23(i)).

The first wire clamper 314 which has been closed is opened and theinsulated wire 313 is pulled back by the tension produced by the backtension mechanism acting thereon in the direction of the reel 312. Theball 313c at the front end of the core wire 313a is caught by the frontend of the capillary 310 and the condition, of FIG. 23(a) is restored.The apparatus is now ready for the next bonding operation.

When the height of the first discharging electrode 316 from the surfaceof the lead 304b is represented by L₁₁ and the discharge gap with thefront end of the insulated wire 313 is represented by L₁₀, it isobtained.

    L.sub.2 =L.sub.9 +L.sub.10 +L.sub.11,

The height L₂ Of the capillary 310 in the step of FIG. 23(i) can beobtained by calculation. Thus, the control unit 320 controls the linearmotor 308 to set up the height L₂ of the capillary 310.

The discharging conditions for the first discharging electrode 316 toform the ball 313c in the present embodiment, larger current and shortertime are preferred than in the discharging conditions for the seconddischarging electrode 317 to form the exposed portion 313d. For example,making the polarity of the discharging electrode 316 negative relativeto the exposed portion 313d, 0.5-2.0 msec of discharging period and30-100 ma of discharge current are used.

By repeating the sequence of processes of FIG. 23(a) to FIG. 23(i), thewire bonding for electric connections by wiring the insulated wire 313between each of the plural bonding pads 303a on the semiconductor chip303 and each of the plural leads 304b corresponding thereto on the leadframe 304 can be performed.

Since sequences of bonding processes are cyclically performed, adetailed description at the start of the foregoing explanation wasomitted for simplicity of explanation. But, in the present embodiment,it is necessary, at the start of the sequences of the bonding operationsfor wiring between each of the plural bonding pads 303a on eachsemiconductor chip 303 and each of the corresponding plural leads 304b,to have, as described above, the exposed portion 313d previously formedat a predetermined distance from the end of the insulated wire 313according to such factors as the distance between the bonding pad 303aand the lead 304b as in the first bonding sequence.

In the present embodiment, a sequence of dummy bonding operations isperformed prior to the actual bonding sequences of operations on eachindividual semiconductor chip 303 according to the above describedsequence of operations of FIG. 23(a)-FIG. 23(j) using such a portion asa side end portion of the lead frame 304, which will be cut off andthrown away at a later sealing process and therefore has no connectionwith the quality of the product. By practicing such dummy bonding, it ismade possible to perform the process to form the exposed portion 313d ofthe core wire 313a at an appropriate distance from the front end of theinsulated wire 313 as shown in FIG. 23(a) according to such factors asthe distance between the bonding pad 303a of the semiconductor chip 303and the lead 304b as the first combination in the actual bondingsequence.

According to the present invention, as described above, the insulatingfilm 313b at a certain distance from the end of the insulated wire 313to be bonded with the lead 304b is previously removed from the insulatedwire 313. Therefore, when the peripheral surface of the insulated wire313 and the lead 304b are bonded, as shown in FIG. 23(d), the insulatingfilm 313b does not come therebetween. The bonding is performed with thecore wire 313a and the lead 304b held in direct contact. Therefore, theinsulating film 313b or such foreign materials produced by thermallycaused change in quality of the insulating film 313b never comes betweenthe core wire 313a and the lead 304b or between the core wire 313a andthe front end of the capillary 310.

Therefore, compared with the bonding in the prior art performed throughthe insulating film 313b, greater bonding strength and lower electricresistance are obtained at the bonded portion. Thus, a good bondingcharacteristic is obtained at the bonded portion between the peripheralsurface of the core wire 313a of the insulated wire 313 and the lead304b.

Thus, it can be prevented, for example, that the insulated wire 313 andthe lead 304b are separated at the bonded portion after being assembledinto a semiconductor integrated circuit device which produces adefective product. Reliability of the quality and operation of thesemiconductor integrated circuit device can be improved.

Further trouble is prevented from occurring when foreign materials enterthe wire insertion hole of the capillary 310 to clog the insertion boleof the capillary for the insulated wire 313 to pass therethrough whichimpairs smooth delivery and pulling of the insulated wire through thecapillary. Since smooth delivery and pulling of the insulated wire 313is performed at all times, stabilized bonding work can be continued fora long period of time.

As the consequence of the foregoing, maintenance of the wire bondingapparatus can be simplified and its operation rate improved, and thus,productivity in the wire bonding process for semiconductor integratedcircuit device can be improved.

Embodiment 3

FIG. 25 is a perspective view of a wire bonding apparatus as anembodiment of the present invention and FIG. 26 is an enlarged sectionalview showing a portion of the same. In the wire bonding apparatus of thethird embodiment, there is provided a second discharging electrode 470for forming the exposed portion 313d at a certain distance from the endof the insulated wire 313. The second discharging electrode 470 is madeof a conducting material such as tungsten (W) and comprises a pair ofdischarging electrodes 470a and 470b opposing each other across theinsulated wire 313 and insulator pieces 470c and 470d made of aninsulating material such as ceramic and fixed to the electrode pieces470a and 470b, respectively, in such a manner that they are projectingfurther than the electrode pieces toward the insulated wire 313. Thus,the insulator pieces 470c and 470d prevent the pair of electrode pieces470a and 470b opposing each other across the insulated wire 313 with apredetermined gap left therebetween from coming into direct contact withthe insulated wire 313. In the present embodiment, the seconddischarging electrode 470 is driven by a discharging electrode drivingmechanism comprising a pivoted shaft of 471 having a plurality of swingarms 473a, 473b which are coaxially supported by the shaft having endswhich are connected to separate solenoids 472a, 472b, respectively, apair of guide arms 474a, 474b, on which bases are fixed to the otherends of the swing arms 473a, 473b and front ends are connected to thepair of electrode pieces 470a, 470b of the second discharging electrode470, and a frame member 475, which is fixed to an X-Y table 305 andsupports the shaft of swing 471 and the plurality of solenoids 472a,472b.

Between each of the swing arms 473a and 473b and the frame member 475,there are interposed springs 476a and 476b, respectively, for providingeach arm with torque constantly urging each arm to rotate in apredetermined direction.

Between the surfaces opposing each other of the end portions of theswing arms 473a and 473b for supporting the guide arms 474a and 474b,there is provided a gap adjusting piece 477 locked to the swing arm473b. The minimum distance between the guide arms 474a and 474b broughtabout by approaching each other through swing displacement of the swingarms 473a and 473b, that is, the minimum distance between the pair ofelectrode pieces 470a and 470b of the second discharging electrode 470,is steadily maintained.

More particularly, when the second discharging electrode 470 is not inuse, the solenoid 472a is relaxed so that the swing arm 473a is rotatedthrough urging force of the spring 476a. The second solenoid 472b isactivated so that the swing arm 473b is rotated against the urging forceof the spring 476b so that the electrode pieces 470a and 470b areretracted through the guide arms 474a and 474b in a direction away fromeach other.

In contrast, when, the solenoid 472a is activated so that the swing arm473a is rotated to a predetermined position against the urging force ofthe spring 476a, its reference position is set up. In the meantime, thesolenoid 472b is relaxed so that the swing arm 473b is rotated throughthe urging force of the spring 476b until it reaches the position wherethe gap adjusting piece 477 comes into abutment with the swing arm 473aon the opposite side, so that distances of the electrode piece 470a andthe insulator pieces 470c, and the electrode piece 470b and theinsulator pieces 470d, of the second discharging electrode 470 from theperipheral surface of the insulated wire 313 are controlled to be apredetermined value with precision.

More particularly, in the case of the present embodiment, byrepresenting the distance between the insulator pieces 470c and 470d byl₁, the projection of each of the insulator pieces 470c and 470d fromeach of the electrode pieces 470a and 470b by l₂, and the diameter ofthe core wire 313a of the insulated wire 313 by l₃ as illustrated inFIG. 26, the discharge gap l₄ is expressed as

    l.sub.2 <l.sub.4 l.sub.2 +(l.sub.1 -l.sub.3)/2.

If, for example, l₂ =0.2 mm, l₁ =0.1 mm, and l₃ =0.03 mm, the dischargegap l₄ can be precisely set from the above expression within the range

    0.02 mm<l.sub.4 <0.235 mm.

In the third embodiment, as described above, the discharge gap l₄ can beprecisely set up when the exposed portion 313d is formed by dischargesproduced between the second discharging electrode 470 and the insulatedwire 313. In addition, shorting between the core wire 313a and theelectrode pieces 470a, 470b is prevented by existence of the insulatorpieces 470c, 470d. Therefore, it is made possible to cause stabledischarges and have the range and position of the exposed portion 313dformed at the specified portion of the insulated wire 313 with highaccuracy.

Embodiment 4

FIG. 27(a) to FIG. 27(h) illustrate a method of wire bonding accordingto a fourth embodiment of the present invention as sequence ofmanufacturing steps. First, the capillary 310 is positioned right abovethe bonding pad 303a of the semiconductor chip 303, and at this time,the insulated wire 313 passed through the capillary 310 is restricted inits movement by the first wire clamper 314, with an end portion of alength necessary for forming a ball 313c projecting out of the front endof the capillary 310 (FIG. 27(a)). A first discharging electrode 460comes under the insulated wire 313 with a predetermined discharge gapleft therebetween and produces a discharge between the same and thefront end of the insulated wire 313 to thereby form the ball 313c (FIG.27(b)). Thereafter the first discharging electrode 460 retreats to oneside, and at the same time, the capillary 310 descends a predetermineddistance with only the first wire clamper 314 closed (FIG. 27(c)). Atthis time, the ball 313c of the insulated wire 313 is not in contactwith the bonding pad 303a of the semiconductor pellet 303 thereunder.Thereafter, the capillary 310 rapidly ascends to a predetermined heightwith the first wire clamper 314 opened, when the insulated wire 313 isbrought into a state where it is delivered from the front end of thecapillary 310 a predetermined length by action of inertia of theinsulated wire 313 itself. The second wire clamper 315 is closed so thatthe insulated wire 313 comes to a stabilized state. Then, the firstdischarging electrode 460 approaches the insulated wire 313 at apredetermined distance from its end from the side with a predetermineddischarge gap left therefrom. Then, a discharge is produced between thefirst discharging electrode 460 and the core wire 313a through theinsulating film 313b. By energy of the discharge at this time, theexposed portion 313d is formed on the insulated wire 313 at its positionat the predetermined distance from its end (FIG. 27(d)). Thereafter, thefirst discharging electrode 460 retreats sideways and the second wireclamper 315 is opened. The insulated wire 313 is returned to the side ofthe reel 312 by action of the tension constantly exerted by the backtension mechanism on the wire so that the ball 313c at its end comes tobe caught by the front end of the capillary 310 (FIG. 27(e)). Then, thecapillary 310 descends onto the bonding pad 303a of the semiconductorchip 303 with both the first wire clamper 314 and the second wireclamper 315 opened and presses the ball 313c against the bonding pad303a under vibration given thereto. Thus, the ball 313c of the insulatedwire 313 is compression bonded to the bonding pad 303a (FIG. 27(f)).Thereafter, the capillary 310 ascends to the height where its front endcoincides with the position of the exposed portion 313d which wasalready formed in the previous step of FIG. 27(d), travels to the side,descends toward the target lead 304b and presses the peripheral surfaceof the core wire 313a exposed at the exposed portion 313d against thesurface of the lead 304b, with ultrasonic vibration applied thereto, tothereby compression bond it to the lead (FIG. 27(g)).

Since with of the present embodiment as with the first embodiment noinsulating film 313b exists between the peripheral surface of the corewire 313a and the surface of the lead 304b, sufficient bonding strengthcan be obtained, and highly reliable bonding work can be performed.Further, there occurs no change in insulating film 313b attaching to thefront end portion of the capillary 310. Then, the capillary 310 iselevated to a height that the length of the insulated wire 313 necessaryfor forming the ball 313c is drawn out and stopped. The first wireclamper 314 is closed so that the insulated wire 313 is prevented frombeing drawn out. The capillary 310 is further elevated so that the frontend portion of the insulated wire 313 is broken at the bonded positionwith the lead 304b (FIG. 27(h)). Then, the capillary 310 is positionedat, a predetermined height right above the bonding pad 303a of thesemiconductor pellet 303 to be bonded in the next position to therebybring the apparatus ready for the next bonding operation. Thus, thecondition illustrated in FIG. 27(a) is restored and the sequence ofbonding operations for one set of bonding pad 303a and lead 304b isfinished.

Since with the present embodiment, the same conditions exist as with thesecond embodiment, no insulating film 313b comes between the peripheralsurface of the core wire 313a and the surface of the lead 304b. As aresult sufficient bonding strength can be obtained and highly reliablebonding work can be performed. Further, since no insulating film 313battaches to the front end portion of the capillary 310, the operationsfor delivering and pulling the insulated wire 313 through the capillary310 are smoothly performed at all times and stabilized bondingoperations can be continued for a long period of time.

Further, with the present embodiment, the length of the insulated wire313 drawn out for forming the exposed portion 313d can be controlled bythe vertical movement of the capillary 310, without making use of thebond of the front end of the insulated wire 313 with the lead 304b.Therefore, the need for the dummy bonding operation as in the case ofthe embodiment 2 can be eliminated.

While the invention has been particularly described with reference topreferred embodiments thereof, it is to be understood that the presentinvention is not limited to the above described embodiments. Variousmodifications may be made in the invention without departing from thespirit thereof.

Effects obtainable from representative aspects of the inventiondescribed above in its preferred embodiment will be described asfollows. The wire bonding method according to the present invention is amethod in which an insulated wire passed through a bonding tool is used,and the operation to bond the front end of the insulated wire to a firstposition and the operation to bond the periphery of the insulated wiredrawn out of the bonding tool to a second position are performed tothereby achieve an electrical connection between the first position andthe second position. In this method, the insulated wire is drawn out ofthe front end of the bonding tool with a required length calculatedbased on information on positions of the first and the second positions,and a discharge is produced between the core wire of the insulated wireand the discharging electrode through the insulating film at the portionof the wire to be bonded to the second position, so that the insulatingfilm is removed by energy of the discharge at that time. The exposedportion of the core wire is formed, and the exposed portion is bonded tothe second position. Therefore, it is made possible to perform thebonding operation with the core wire exposed at the exposed portion andthe second position held in direct contact. Hence, lowering of thereliability on the bonded portion due to the insulating film materialcoming between the core wire and the second position is eliminated. Thereliability of the bonded position between the periphery of theinsulated wire and the second position is increased.

Further, since the insulating film does not come between the core wireand the second position, any foreign material resulting from peeling offand thermally produced change in the quality of the insulating film aregreatly reduced. Since the bonding tool presses the core wire of theinsulated wire exposed at the exposed portion directly to the secondposition, no foreign materials enter the bonding tool. Smooth passing ofthe insulated wire through the bonding tool is assured and stabilizedcontinuation of the bonding operations is made possible.

The apparatus for wire bonding according to the present inventioncomprises a bonding tool, through which an insulated wire is passed andwhich makes three dimensional movement relative to an aimed target, anda discharging electrode producing necessary discharges between theelectrode and the insulated wire. By performing a process to bond thefront end of the insulated wire where a ball has been formed by thedischarge to a first position and a process to bond a peripheral portionof the insulated wire drawn out of the bonding tool to a secondposition, an electrical connection between the first position and thesecond position is achieved. In this apparatus, the length of theinsulated wire to be wired between the first and second positions iscalculated based on information on positions of the first and secondpositions. The required length of the insulated wire is drawn out of thefront end of the bonding tool based on the result of the calculation. Anexposed portion of the core wire is formed at the portion to be bondedto the second position by producing a discharge between the core wire ofthe insulated wire and the discharging electrode to eliminate theinsulating film by energy of the discharge at that time. This exposedportion is bonded to the second position. Therefore, by suitablycontrolling first and second independently movable clamps, the drawingout of the insulated wire from the bonding tool is controlled. Theexposed portion of the core wire can be formed by removing theinsulating film at the portion to be bonded to the second position atany desired distance from the end of the insulated wire prior to thebonding of the peripheral portion of the insulated wire to the secondposition. Thus, the bonding operation can be performed with the corewire exposed at the exposed portion and the second position placed intodirect, contact. Hence, lowering of the reliability of the bondedportion due to the insulating film coming between the core wire and thesecond position is eliminated and the reliability of the bonded portionbetween the periphery of the insulated wire and the second position isimproved.

Further, since no insulating film comes between the core wire and thesecond position, production of foreign materials resulting from peelingoff or thermally caused change in quality of the insulating film can begreatly reduced. In addition, since the bonding tool can press the corewire of the insulated wire exposed at the exposed portion formed inadvance directly to the second position, no foreign material gets intothe bonding tool. Smooth passing of the insulated wire through thebonding tool is assured and stabilized continuation of the bondingoperations is made possible.

What is claimed is:
 1. An insulation film coated wire bonding method forrepeating steps of producing a discharge through a predetermineddischarge gap between a front end of a first portion of an insulationfilm coated wire wound on a wire spool and drawn out through a bondingcapillary and a discharge electrode by an output voltage of a dischargevoltage generator for producing a ball on the front end of the wire, andthen pressing the ball against a predetermined bonding position by thebonding capillary to ball bond the first portion of the wire to a firstposition on a semiconductor device chip, and subsequently wedge bondinga second portion of the wire to a second position outside thesemiconductor device chip by the bonding capillary comprising:(a) priorto a discharge for the ball formation of the ball bonding electricallydetermining a voltage drop variation or an electrical parametercorresponding to the voltages drop variation due to a decrease in lengthof the insulation film coated wire wound on the spool at a discharge fora preceding ball formation; and then (b) controlling the output voltageof the discharge voltage generator in accordance with the predeterminedvoltage drop variation or electrical parameter to maintain an appliedvoltage across the discharge gap between the discharge electrode and thefront end of the wire substantially at a predetermined value at thedischarge for the ball formation of the ball bonding.
 2. A method forwire bonding according to claim 1 further comprising:prior to the wedgebonding removing the insulation film of the second portion of the wireby a discharge between the second portion of the wire and the dischargeelectrode by an output voltage of the discharge generator.
 3. Aninsulation film coated wire bonding method for repeating the steps ofproducing a discharge through a predetermined discharge gap between afront end of a first portion of an insulation film coated wire wound ona wire spool and drawn out through a bonding capillary and a dischargeelectrode by an output voltage of a discharge voltage generator forproducing a ball on the front end of the wire, and then pressing theball against a predetermined bonding position by the bonding capillaryto ball bond the first portion of the wire to a first position on asemiconductor device chip, removing the insulation film of a secondportion of the wire by a discharge between the second portion of thewire and the discharge electrode by an output voltage of the dischargevoltage generator, and subsequently wedge bonding the second portion ofthe wire to a second position outside the semiconductor device chip bythe bonding capillary comprising:(a) prior to a discharge for theremoval of the insulation of the wire electrically determining a voltagedrop variation or an electrical parameter corresponding to the voltagedrop variation due to a decrease in length of the insulation film coatedwire wound on the spool at a discharge for a preceding insulationremoval; and then (b) controlling an output voltage of the dischargevoltage generator in accordance with the determined voltage dropvariation or the electrical parameter to keep an applied voltage acrossthe discharge gap between the discharge electrode and the first orsecond portion of the wire at a predetermined value at the discharge forthe ball or wedge bonding.
 4. An insulation film coated wire bondingmethod for repeating the steps of producing a discharge through apredetermined discharge gap between a front end of a first portion of aninsulation film coated wire wound on a wire spool and drawn out througha bonding capillary and a discharge electrode by an output voltage of adischarge voltage generator for producing a ball on the front end of thewire pressing the ball against a predetermined bonding position by thebonding capillary to ball bond the first portion of the wire to thefirst position on a semiconductor device chip, removing the insulationfilm of a second portion of the wire by a discharge between the secondportion of the wire and the discharge electrode by an output voltage ofthe discharge voltage generator, and subsequently wedge bonding a secondportion of the wire to a second position outside the semiconductordevice chip by the bonding capillary comprising:(a) prior to a dischargefor the removal of the insulation of the wire determining a voltage dropvariation or electrical parameter corresponding to the voltage dropvariation due to a decrease in length of the insulation film coated wirewound on the spool by forming a closed circuit of a length of the wireon the spool through output terminals of the discharge voltage generatorand applying a voltage to the closed circuit; and then (b) controllingthe output voltage of the discharge voltage generator in accordance withthe predetermined voltage drop variation or electrical parametercorresponding thereto to keep an applied voltage across the dischargegap between the discharge electrode and the first or second portion ofthe wire substantially at a predetermined value at a discharge for ballor wedge bonding.